there is a link on BMW world somewhere to a guy who turbocharged his M10 powered E30. He custom fabbed almost all of it, including an intake manifold that looked like a hacked together set of headers, but he was getting i think something like 200 horsepower from it.
It's possible on M10 because of the relatively low compression. With some valvetrain enhancement, the M20 Eta would be ideal for forced induction.
A BMW is a beautiful car from almost any angle, but none so much as the one from behind the wheel.