I would imagine quite a bit of cutomizing. The shape of the body and body panels wouldnt line up, at least that'* my guess. Same deal with trying to do a 92-95 body on a 96-99. They look identical but the lines just wouldnt match up.
Actually seeing as how the SE didn't change since the '00 redesign, I think that the GXP panels would be a direct fit for all 00+ B'villes. The only thing they really changed are the bumpers and ground effects. I'm sure that the doors underneith the groundeffects (which are held on by double sided tape on the doors) are the exact same as used in the '04 & '05 SE'*. I also couldn't imaging GM making the fenders & inner bumpers (crash reinforcements) any different between trim levels either.
Going from 92-95 & 96-99 is the same as trying to fit something from a 96-99 to a 00+. Different body styles all together.
I like the looks of the GXP and all, but the only thing that I think I would take from a GXP to put on my SSEi are the tail lights including the center section... But I can clear them mine just as easily, so those aren't really all that necessary.
I wonder how the lower section of the ground effects would look on an SSEi while keeping the upper sections on the doors? That would help to keep the side panels shiny instead of all of the road debri making them dull looking...(especially the kick out right in front of the rear wheels...)