I'm not expert so please correct if i'm wrong,
Its probably the atmospheric
BOV thats causing the backfiring (It's not an issue)
I believe it caused by an excess of fuel in the system, this occurs when the
BOV releases air into the atmosphere it causes a temporary unbalance in the air/fuel ratio being combusted. With it being slightly richer thats when you'll find it popping and backfiring.
I have an 05 Sti with indentical mods, and its occured once or twice when i missed a gear whilst reving it out hard.
Having an 'atmospherioc
BOV' will increase your fuel consumption slightly due to these unbalanced ratio's. Alot of people would suggest switching it back to the standard plumb back model (which i would if i could click my fingers and have it changed). The only beneficial/reasons why people put them as far as im aware is
1) They like the sound it makes
2) Slightly improved throttle response (i think due to temporary richness after shift)
3) I've read the standard ones CAN start to leak around 22ish psi?
From my understanding it isnt necessary at all unless chasing big power. and i would remove it next time you were getting you car tuned.
Again this is all information i've read online (unfortunately a lot of online information is opinion and wrong) so if im off target with anything here some one correct me.