850cc is actually way too big. Unless you've got an ECU (read Motec or Autronic) which is capable of controlling injectors at very small openings then you'll find it quite difficult to maintain idle with these large injectors. You also have the issue of having to run ballasts as some ECUs cant drive these 800+cc injector which are low impedance/high current.

The way I figure it, 440cc injectors are good for around 180-200kw atw at around 80-90% IDC. Double the injector size, you'd be wanting to have around 360-400kw atw at the same IDC. Not many WRXs are pushing 200kw let alone 250kw.

Upto 250kw use 550cc
Upto 300kw use 650cc or higher