In standard form the 2.5 isn't very fast, not to many cars from the 80's are by todays standards. However there are turbo/supercharger kits available from a number of manufactors or you can turbo/supercharge them yourself. A displacement of 2.7 can be achived quite easily...plus some other bits and your the right side of 200hp.

m50/m52's (2.5 or 2.8 24v engines) from e36's or late e34's are also a common engine swap as is the 3.5 stright 6 m30 engine. Not forgetting the motorsport engines from the e36 m3 and also the e28/e35 m5 engine...depends how much you wanna spend, if they get boring you can then turbo/supercharge them!

http://www.pistonheads.co.uk/sales/1081579.htm

On the M5board there is a user putting a v10 m5 engine in the E30