I see no benefit to getting the GXP if you're not getting a manual....for one you will have more choices looking for a GT vs. a GXP because more were produced. W/ a tune/intake it is very close to GXP power - My car made 332whp on a mustang dyno w/ just intake/custom tune/axleback - that's about 405HP @ the motor - very close to a GXP. While the GXP runs bigger calipers which will perform better during track days/autocross - the GT's brakes are phenomenal as well and very good for the street....no matter which car you get you WILL be needing to change brakes in the future so you can always upgrade lines/rotors/pads on the GT to heighten performance...its a "maintenance mod" when you think about it.
I don't regret not getting a GXP, I only miss having a manual, if I was buying a used G8 it'd be the GXP ONLY because of the manual. The GT's can be made as fast pretty easily and the gas mileage is a bonus...they ARE better around town than the GXP's too if you're wondering.
Save the extra money get the color combo/mileage you want in a GT since there are more out there you will have a better chance of getting the year/mileage/color/options you desire, then w/ that extra coin throw on headers/intake/tune and you'll have a faster ride than a GXP for less money w/ some custom touches.
Can't go wrong either way really though...enjoy
'08 GT sport/premium, tints 20/35, 18" TSW nogaros w/ bridgestone blizzak winter, 19" sport oem w/ hankook evo summer, Philips crystalvision bulbs, debadged, Vararam OTR, JBA 1-3/4" LT's, ported TB, MF axlebacks, MF X-pipe, DBA-powerslot brakes, russel SS lines, hawk ceramic pads, GXP shocks/struts, lovells poly strut mounts, Slowhawk Tuned 344/356
Gone -'04 GTO A4 2.5" custom exhaust, sweet thunder muffs, H pipe, Z06 heads, Baer brake it, 20% tints, torrid red, AEM brute force intake, tuned