I also agree with what you said in your original post regarding the potential of these cars with some decent tires and a more track oriented suspension.
For example, my best time in the 335i at Roebling was two months ago on the original runflats at a 1:26.9. My old E36 M3 (240hp) also did about 1:27's stock, but with coilovers and r-compound tires did a high 1:22, and could turn 1:23's and low 1:24's all day.
With some R-compound tires and some decent coilovers with camber plates I think the 135i/335i would surprise some folks. Of course then you would need a "real" LSD, a race seat to hold you in place, etc, etc. It's a slipery slope....I am staying dead stock
