haha
all of those crosslace wheels are 8" wide wth 25mm offset, so they aren't really TGP specific. if i were going to add a spoiler to a TGP, i would probably find a different trunklid and put the spoiler on that one, then store the TGP trunklid. the only reason is because of the stickers etc on the inside of the trunklid i guess.
That and its probably easier (and cheaper) than plugging the holes if you decide to change it back.
I have a friend in London and in Waterloo/London (Q-ball and GTP_Munky on the forums) who have 92 and 95 GP GPs, and one of them (I forget) are thinking of ditching the spoiler and going bareback, so you might wanna contact them about trading decklids. I really would keep a TGP as original as possible, of course swapping in a 5-speed really is so much fun to pass up if that's your kinda thing.