Sorry Gary, I beg to differ. DAA- I've tested my exhaust a fair amount in all forms with the removal of the factory muffler, removal of the 3rd cat, install of spintech muffler, ran both with and without the 3rd muffler with that. Also with clogged front upper cat, and full replacement with both new cats, new 3rd cat and spintech. I'm currently running all 3 cats and the spintech now.
Having done so with the changes all on MY rig with no other changes at the time, I will say this-the biggest increase you'll see in power, performance and yes, Mileage-will come from pulling the factory muffler and swapping in a high-flow setup like the spintech or other high flow setup, and the removal of the 3rd cat. That will be your biggest gain in performance/power/mileage overall, BUT at the expense of added noise. I'm guessing this will be TOO loud for you. It's as loud as I will tolerate myself. I also like a quiet rig through all the ranges and especially at slow-crawl on the trail.
My exhaust guy here in town did say that if it was too noisey for me, he could install either a second high-flow muffler in front of my main muffler (where the 3rd cat is now) or another small chamber device to help isolate the sound a little and said it would make a noticeable difference in sound (quieter) without the drawback of decreased performance. I never did have this done, although I wish I would have tried it just for kicks. When my upper front cat clogged, I got a full replacement from Eastern Catalytic, which included the 3rd cat. I installed this all myself and welded it in, to the existing spintech. My exhaust guy had made 3 prior attempts years ago to relocate the exhaust tip due to hitting it off-road a lot. It took 3 attempts as each change he'd make, I'd go wheel it and find where my suspension would make contact and in what situations, so I'd go back and explain that to him, he'd re-route it again and I'd test again. After the 3rd time, we found a setup that would work for me without any suspension contact, or contact off-road. He did this all at no charge to me, just said to come back with some "real" work. I took him a case of beer and a crisp bill as a Thank You which he was grateful for. I've been recommending him ever since and sending him lots of business. He's about the best in town and is GREAT at what he does. It's worked well for both of us. I kept his rear tailpipe section in place as it works well with my current setup.
Now, with the setup I currently have in place-the new Eastern Cat full replacement setup including 3rd cat and spintech, I WILL be removing the 3rd again as I did see both a drop in mileage and power/performance with the addition of the 3rd again. I'm not happy with that with the towing and wheeling I'm doing and want that slight added performance/mileage back. I think this time I WILL try that addition of the second high-flow muffler or other chamber to see if it effects power at all and what it does to the sound levels.
One other test I did do was a decible sound test running my rig using a hand-held decible reader with a few of the mods. To my surprise, the spintech setup and removal of 3rd cat actually came in a decible or two quieter than the stock setup at various points around the rig and inside the cab. Not sure why that was, I tried several tests with and without using the same meter. I'm not a sound guy so I can't explain it. It seems louder to the ear, but the meter said otherwise. Perhaps it's the deeper, throatier note you hear, I don't know. Regardless, there was no "droning" with any of the above setups. I've heard that before in other rigs and in some friends Jeeps and it drove me crazy! What I might do as the Eastern setup uses ceramic in their cats rather than steel like the factory does, I MIGHT just pull my cat and clean it out or remove the insides and see how that does as the second "chamber" of sorts just for kicks. Haven't had the time lately or physical ability to do much of that type of work just yet. Working on it.
Here's my disclaimer-if you even consider removal of the 3rd cat, as far as the computer issues are concerned, the two upper cats are monitored with O2 sensors. The third cat is not and makes no difference to the computer or running conditions if it's there or not-the computer doesn't know its there so it won't know if it's gone. I never tripped any codes for it's removal for the entire time it was out. There ARE however various laws against modification or removal of stock/factory cats and equipment for emissions reasons. Check your state laws if you do consider this as it may not be a "legal" mod where you're located. I'm not advocating you do this or telling you to do it, just what I've found in my testing.
Now for performance-the best increase I had made a noticeable difference in town. This was the second biggest performance increase and mileage increase compared to the gear swap. It made a big difference, even my wife commented many times on how much more "fun" to drive it was as it just flat up and went much quicker, responded much quicker, shifting the 6 speed changed as well as I could pull hills in town easily in 4th now where I'd need 3rd before at high rpms. Slight differences like that were what I noticed the most. Towing the trailer was easier as well in many areas of the state, for instance on the way up to our ranch, I'd drop to 4th and have trouble maintaining speeds depending on how loaded the trailer was. With the changes, I could run 5th and have power to spare, maintaining higher speeds. Now for fuel economy-with the total removal of 3rd and spintech, I forget the exact numbers but want to say it was a 2-3mpg increase in mileage. It was much bigger than I expected to get and for a while, I was edging on 19mpg out of my heavy rig. 18 was an easy average, 19's were there on the low end and I was working towards trying to get a solid 19 average. Unfortunately never happened as I started getting my OPDG issues and poor running conditions with the #2 misfire. To date, I've been unable to repair that issue and consequently, my mileage is in the toilet. I'm bouncing between 12-15 now with the current engine troubles. One of my maintenance guys at work gave me a couple ideas last night that I will try for kicks as it won't be too much money involved. If that doesn't work, I fear the lifter failure that seems to be more common these days and a 2nd full rebuild. I'm currently on the 3rd OPDG.
For what you're trying to do, I'd recommend a good high-flow muffler at first, keeping the 3rd in place for sound reasons. If you are willing to do some testing of your own, check on your laws and if OK or you feel comfortable with it, remove the third and install either a second high-flow setup or other "sound chamber" of sorts to help dial out the sound. The factory cat is compossed of a steel corrugated cardboard type setup that will be very difficult to remove or "gut out" if you wanted so I wouldn't waste the time with that. I mentioned the ceramic stuff in the one I have now-it will come out easy enough and I might try it on mine as I will be either eliminating it and going back to strait pipe or trying another sound chamber in front to quiet it down a bit. I'm afraid the strait pipe to high flow muffler will be too loud for your liking. Also, with regards to having a "shop" do it, if you did request a removal, I highly doubt most "big" shops would be willing to do that due to regulations. You might try and find a smaller owner-operated shop you can negotiate things with a little easier. Even without, I do prefer the smaller owner/operator shops as opposed to the bigger chain outlets but that's just me.
Hope that helps.
Best of Luck,