It'll work just fine. But it's not ideal because your not addressing ignition. So you can add or subtract as much fuel as you desire, but the OE computer will pull back timing and you'll lose power.

Now if you had an STi engine and ECU, an S-AFC would do wonders as it's ECU is less prone to pulling back timing.

 

or you can get vishnu's emi adapter to help with the detuning issue.