I don't know if F1 Cups will clear your brakes. The brembos on the STi are quite deep. You should check that but if they're similar to the Ultraleggera, you might be fine.
The 2011 STi is 5x114.3 remember, I don't remember what the 3000GT was but I have a memory of 5x100? If so, they're not compatible.
265/35R18 is a good size. There's a rumor you can even fit 275/35R18 at +55 and still clear, but I'm not so sure. Especially at +55 or +56. 265 is a lot wider than stock already.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD