2.5 complete exhaust VS a 3.0 complete exhaust

Honestly, in terms of OP, the 2.5 system is more than adequate for you needs, and cheaper, and lighter.
In terms of science... I think the reason you don;t see huge dyno results overall is simple: Every package is unique, and by package, I mean the chassis, engine, noise level acceptance, etc. I bent custom pipes for a couple years. Xs and Hs will quiet any exhaust. So will turning down the ends, and going from larer diameter to smaller diameter piping, adding bends, changing the position and size of the muffler or adding a resonator.... I think the main issue is with a V8 layout, X can't be placed where it might actually help.. which would be closer to the collectors. The trans and crossmember is in the way. Look at a Winston Cup car on it's roof... They do have merged pipes, and they are way rear of the trans. I'm fairly sure that's because it makes power, and they are tuned for enhancing peak power which in those cars is 8-9K rpms. It would be lighter to have them exit without traversing the chassis.