It seems to me that you would want to do things that both add performance and subtract weight. The comments about wheels and tires are spot on! But also think about a lighter exhaust system, battery, brake rotors and maybe taking the trunk carpet out.
Short of stripping the car for dedicated track duty (not a bad idea - get a Hurricane Sandy car, strip it, cage it, stand-alone ECU...!) I think those ideas are about as far as we can go and still maintain the car as a DD.
If you were really serious, I guess you could swap the bottom seat frame for the manually adjustable seat from the euro 316i. Getting rid of the adjuster motors would probably save quite a few pounds.