Well, alot of guys are doing this, and with great results.
But i've come across some issues however.
Steptronic gear boxes are breaking down for instance,
In the 184 hk/390 Nm engine (M57), BMW has used a General Motors box A5S390R.
And this transmisson can only deal with 390 Nm, the same as the engine produces...



So i think you need to look at what kind of engine you have, and then decide to upgrade you're engine...