fiveoh

Most GTs with the mods he listed are making 400-440 not 450. With heads milled/valvejob you can get 450.

GXP I would assume 450.

Sticks n Stones

GXP has the full blown LS3, GT has a L76 6.0L: essentially a LS2 short block with LS3 heads and displacement on demand (removeable). If you think that LS2's with the LS3 heads cant make over 450rwhp, then argue with all these guys:
http://www.ls1gto.com/forums/showthread.php?t=286447

Name rwhp/tq trans cam
PhillyGoat........486/435... M6... 232/238
Johnehale2003..485/439... M6... 231/234
SAjenkins24....479/430... M6... 232/236
kanibal...........477/462... A4... 224/230
Tims05...........476/416... A4... Cartek Custom
goatryder08....468/439... M6... 216/229
ArthurJGuy.....461/421... M6... 234/245
DaGoat6Litre...459/436... M6... 222/224
skyfz_450.......458/430... M6... 228/240
Badimpala.......458/421... M6... 227/239
KC2GIW.........456/427... M6... 228/240
Stevo...........452/419... M6... 224/240
TOCgoat........452/412... M6... 228/244
PBANDPKGTO..451/403... A4... 232/236
CDRUM1.........450/400... M6... ??

There is alot more who havent been added or have modified bottom ends, running boost, etc.