BMW parts swap too you know. Do some research on 1addicts.
Basic mods:
All the underpinnings to an M3 fit on a 135i, camber giving wishbones, better bushes, light weight stuff, ARBs... Add a diff from Birds and you've got a 1M.. Only for 20k less. St1 map = 380bhp and similar torque, easy 400s with downpipes and intercooler.
Ditching run flats and moving up to a 225 on the front does wonders. Any of the above is just M3 chasing.
Oh, and they're much faster than their book figures