OP, the suspension and tires are definitely the weak links in the 135i's performance. The engine can be upgraded fairly easily with a tune and bolt ons. I've spent most of my time (and money) so far on the suspension and wheels and tires.
I highly recommend the M3 suspension upgrades along with aftermarket springs/dampers/coilovers. In the front I've added an M3 (E93) FSB, M3 control arms and tension rods and Koni Sport dampers. In the rear I've added Koni Sport shocks, 2011 M Sport linear springs and M3 guide rods. Next I'm planning on adding the M3 subframe bushings.
The suspension mods coupled with the non runflat tires really transforms the way the car handles.
2009 BMW LeMans Blue 135i/Dinan Stage 2 tune/Alufelgen SF-71s/M3 suspension components/Koni Sport adjustable dampers/ETS FMIC/M3 paddle shifters/Alpina B3 TCU flash