Tried to respond but it wouldn't work, so posting it here
Bavarian auto has a few good alternatives bavauto.com tuning a beemer isn't going to be like your hondas in almost any regard. Doing the math selecting the RIGHT turbo is everything. Overboosting will kill the head gasket nothing flat and an overheat can destroy an engine, even one time only. BMW designs these things to give just about every ounce of power they can give right out of the box. Trying to squeeze more gets REAL expensive real fast. I do know that in its stock form my 87 325is has eaten many a tuner honda live. The inline 6 gives uber torque for its displacement low in the rpm band and the car will wind to 8g's real fast under load as a result. Most small 4 bangers don't make any real torque or power until 6-8 grand and by then the race is over. Try it stock and with stock exhaust for a bit (big pipes rob torque and therefore power) you may find that only a few mods (like a flowmaster muffler only and a chip) is all you really need to beat your buddy's car (and it'll eat him alive because you're doing it with way less.....on paper at least). Also cold air intakes, generally a waste of time on these. They already have them as well as true dual exhaust, in a 6 cyl. Haven't heard of az bimmer motor werks. There was a bills beemers over on mcclintock, they royally pissed me off once and I never went back. Do all of my own work as a result.