Unfortunately you do not have an M103 engine, you have an M104 Engine, a 3.0 litre 24 valve inline 6 that produces 217 hp and around 220 ftlbs of torque.
TO boost performance, I would start by searching on the site for mods.
If you can find a chip, then it will help a little. A good cat back free flow exhaust will do nicely. a Modified Airbox, designed for more flow, porting and polishing, and a jacobs or other electronic ignition upgrade, will probably help just a little. all this may or may not give you many gains in hp, but you will get some.
Other than that there is not much you can do with an engine that is pretty much tweaked from the factory, unless you go with some more major mods, like boring and putting in hotter cam, bored out throttle bodies etc, all of which are tough to find or have to be custom made.
you can probably find a good turbo system from mossleman for your car to really give it some oomf, or possibly a supercharger.
you can change your differential for a 3.27 as found in the 260e and the wagon, but you will need to change the speedo to match or have it recalibrated, and you will have to have the tranny adjusted for the new differential. This will give you better accel at the loss of some top speed.
Otherwise, go all out and find yourself a C36 complete donor car, and transplant the engine, computers, tranny, etc and have youself my dream coupe.
hehe
For me, I'll stick to suspension, cosmetic, and other minor mods, then one day, a 500e engine, or a c36 engine will be going in.
hehe
Alon