3" on a GTP with no res or muffs sounds sick as heck.
Every GP I see at the track (rare) they are running 3" straight pipe and usually cat-less.
Very loud and throaty. ALMOST V8 like. ALMOST!
Personally, if 2.5" can be gotten for under $200, I'd go for it. I payed $250 for my 2.5" cat-back with no rez. I even had them leave the muffler on with clamps so I can take it off for the strip.
I'm considering taking it off tomorrow and just putting some spare 2.5" pipe on the end. I'm curious to see how it sounds.