I have no clue on a BMW
We ported my friends Camaro Heads, and holy crap the difference was amazing.
It totally woke up the engine.
You get better top end power, but the best part was low and mid reange power was alot better along with thottle responce.
I don't know how much it would cos though, that really is the question for you, adn that I don't know.
If its not too bad, I'd do it!
318I convertible, Dinan Exhaust and performance chip
Roh BMW racing 17's, Dinan Stage 2 suspension system. Pinoeer CD player, pioneer speakers front and rear. Pioneer 400 watt 6 channel amp. and MTX 8" subwoofer.