Thats low boost for a t34 runnng greens really. Although for 12 psi 309 bhp is very good. Most stage 3 cossies run 25ish psi. Also are your green injectors 803's (dark green) or light greens.

Dont turn the boost up willy nilly without getting the fuelling checked by a pro. there may well be a reason your boost is only 12 psi