You can push over 750 horsepower through a dual 2.5" system with good mufflers. (Flowmasters are NOT one of them!!!!) How much power are you making? The need for a 3.5" is pretty crazy. I personally HATE the sound of a single 3" or 3.5" system. I have built both for other people, and really think they sound like a GAY rabid bumble bee.
But if that is the sound you are looking for, go for it.
I prefere the three muffler system. VERY quiet and VERY fast. A local racer has gone 9.16@149+ through 3 3"mufflers (2-Ultraflows and a modded ATR 3" crossflow) When the down pipe was uncapped on the chassis dyno it only gained SIX (6) horsepower! So......if you want to annoy other folks, go with a single 3.5". I prefere to sneek up on em, and REALLY hurt them.
It hurts them that much more to be beat by a "Singer sewing machine". THAT quiet. Alot of fun. (search "3 muffler system") As they say....."If you can hear the fuel pump over the sound of the engine.......DON'T mess with 'em!"