I've breezed through all the comments and I'll just add my experience with two 110" bikes.
I bought a new '08 SERK Annie and on that I did True Duals V&H. Remember that was the year prior to the under chassis crossover exhaust. I had V&H Wide Ovals, not their BIG Ovals. The sound was VERY mellow and deep, but quite loud. They would settle down a bit on the highway, but they did sound very nice to my ears.
On the '09 SERK I went new with Fullsac 2" and their dyno proven TTS Mastertune and map. This has always ran great and I get very good mileage. No decel pop or hesitation anywhere. The sound was okay, but not very loud. It's deep, but not nearly as mellow at the V&H Wide Ovals. My wife wanted a bit louder.
I found a set of Fullsac 2.5" for sale and bought them. I had read all the stories about loss of power and low end torque, but did it anyway.
I find them still not as loud as the V&H Wide Ovals, but close. They are NOT as mellow as the V&H, but do sound pretty good. Also I have de-catted the CAT and that's all I run is the de-catted stock header pipe.
To be very honest, I've not noticed ANY remote loss of power or even snappiness to the bike. I did lose about 1 to 2 mpg, but that's very subjective to how I ride on any given day.
I would not hesitate again to do to the 2.5", which are no longer made, or the 2.25". They bark on the throttle and quiet down very nicely on highway cruising. I can hear my tunes without any issues. I'm very happy with the performance for my riding tastes.