IMO, heres the deal: If you like the look of the stock header and muffler combination, then there is no question that the Fullsac Stage 1 with the 2" cores is the way to go. The 2" cores will give you slightly more power than the 1.75 or 2.25...note the word slightly. The loudness factor is subjective...totally. The 2" cores are plenty loud for me...in fact, I'd probably like the 1.75's just fine. The Fullsac system just happens to be the least expensive way as well, but it's not CHEAP by any means. Steve works hard to earn trust on many of the HD forums, and he makes a really good product, with excellent support. He has also worked very hard on his canned tunes for that setup, and it will be so close to right that you may not even need to bother with a tuning session elsewhere. Mine runs fine with the canned map. It might run slightly better with a $250 tune, but I'm happy.
Typical numbers for the Fullsac system are going to be 92-96HP and 108-112 TQ. Different Dynos will give different numbers, but an honest set of numbers is probably 94/110. You can hear the stereo at cruising speeds.
If you don't like the looks of the stock system, all bets are off. A two into one pipe will generally give the best absolute numbers, if that's what's important to you. I personally think a dresser looks weird with one mufflers under the bags, but that's just me, and you can always get a ghost pipe. In MOST cases, a true dual header will not give as much TQ down where you ride, but of course, there are exceptions (though none come to mind).So you can spend around $1100 and do the Fullsac, TTS, and cores of your choice. That's if you put them on yourself and can operate a laptop and a Dremel tool. Or, you can spend more and get mufflers and headers that you like the sound/looks of more. I wouldn't give you $.10 for the difference in seat of the pants performance between any of your choices. If you can actually feel 2-3 ft lbs of TQ when riding, I'm not going to dispute your perceptions. What I think is felt is WHERE the TQ comes in...down low where you actually ride a touring bike, or up higher if you wind it up all the time. I personally could GAS what the bike does above 4500 RPM...not that I never go there, but just that I don't go there enough to give up the low end, which is typically the case. If you street race your bike, then obviously you have different needs. The only way to know for sure what will look and sound best for you is to actually look and hear each one on a 110 CVO bike. Everything else is a best guess, or someone's opinion...this is mine.