they won't work in the door, even if you could cut it out large enough, and even if you got a spacer, you still have the door panel. The door panel barely clears a 6.5" driver with a 1" spacer because it curves in right outside the edge of the speaker, there's no way it would fit over an 8. And there's no turning back, because once you cut the hole larger you're screwed. You could build a huge spacer that went all the way out and mounted to the door panel over your 8" hole, but then the door panel wouldn't be able to go back over the spacer. Really a PITA, but the rear deck is a possibility.
'12 Grey WRX - the daily
'05 Silver WRX STi - the toy
'01 Sentra SE - the ready-to-sell