just fyi my car has 140k on it and is still on the origional tranny(gs). the only thing i found in any reciepts was the guy saying it was giving him a clunk when shifting, they replaced the valve body.
granted i did buy it off an old couple that i'm sure did not beat on it but thats still a lot of miles. honestly if it makes it to 160k i'll think its served its purpose and rebuild the tranny just so i dont have to deal with it going out on me. and i'd like to start throwing some performance stuff into it...
i believe the gxp is still the l67 (older supercharged engine, series 2 3800, not a 2004) with some little stuff done to it. i know SLP did a gsx, thats a pretty cool car. there were 3 stage packages you could buy. from basically stock, different wheels, wood trim interior parts, embroderd head rests. up to a smaller pully, exhaust, a good amount of stuff done to the car actually. i'd guess the gxp is the same thing. i'm almost positive they aren't a northstar.