This is kind of an interesting thread, but the topic has been discussed (here, and on E90Post). Ever since
E30ROB totaled his 128i,
desertman123 has the most powerful non-race-spec N52 on the forums.
Best power combo, sticking with an N51/N52 128i (will put you at approximately 250hp at the wheels):
-Active Autowerke Catless Headers
https://store.activeautowerke.com/co...exhaust-header
-N54 Intake Manifold (Part Number 11617564679)
-MILV's (Thread on here...use the search tool or google)
-Tune
-Manual Swap? (less power loss)
If you want any more than that (say, with a supercharger) it really is best to move on to the N54 platform. They already have the stronger transmission (seriously, those things are bulletproof transmissions, and I want one in my car), reinforced driveshaft, axles, differential, clutch, hubs, brakes, etc. To bring a 128i up to such snuff would cost at least as much as a complete, high-mileage 135i, which has an engine that can reliably hold 800hp stock. All the appearance parts on your car will bolt right up, and you'll be off to the races (and maybe winning some). Plus, the torque from the turbo motor will be better for turning those gigantic wheels (backing-down to 17's would make the car feel exponentially quicker
)
Edit: +1 on driver mod! Best thing you can do. Remember, the weakest part of any car is the nut behind the wheel...