A buddy of mine had a Vortech GT, 8 or 9 psi... made 360rwhp. The tails weren't restricting it, although he had a 2.5" H and a set of Flow 40 series welded in.
I seriously doubt it will bottle neck you. I am making over 300rwhp with a SINGLE 2.5" crush bent exhaust on my 88 mustang. That's around 360hp at the crank? Granted, it is obviously restrictive at my level because when I open the exhaust cut out, I gain 2mph in the quarter. So double my single exhaust numbers and you can figure your dual 2.5" system will get restrictive around 600+rwhp (700 at the crank). I think you'll be fine.
I have a friend with a 78 Malibu, street car, with a 406 stroker, fully built, with a nice shot of nitrous and all the supporting mods. He has dual 2.5 inch pipes with a set of Flowmasters that are quieter than mine. I'm not sure about horsepower numbers, but he ran 6.20s in the 1/8 a couple weeks ago.
Restrictive? Maybe.
Necessary to have big exhaust to run good times: definitely not.