I am not a fan of putting different engines is E30s either but their is one transplant that is very interesting. How about putting a 2.8 liter M52 (the one from the Z3)? This engine is lighter than the regular 328is engine, due to the extensive use of aluminum. I am not sure if the Z3 M roadster's 3.2 liter is aluminum also, but if it is than that sounds even better. The E36 M3's S52 3.2 liter engine is heavier than your stock engine, but if you use the Z3's M52 you will retain the 50-50 weight for perfect balance. I would definately stay away from the M5 engine. That engine is too heavy for our cars and the gain in straight line speed and acceleration will not compensate for the bad handling. In esscence you will be left with a drag racer.