Be aware that whichever size Monza you have, the steel spare wheel will not fit over the AP's.
If you have the factory fitted 18" Monza IIs, then you also get an 18" ET-matched spare - so it should fit. 
....Good point, but my GTI came with factory fitted Monza 17's.
OK, so does that mean your space saver is also a 17" ??
Righty, just found the part number for the 18" space saver - 1K0 601 027 B - for a 3.5Jx18H2 ET25.5 steel space saver. The correct tyre is a T125/70 R18 99M.
Edit: and the larger tool holder - 8P0 012 109 E
HTH