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I don't have an M135i, but I have driven the F20 on quite a few track days. My current 118i has the same brakes as the M135i.
The brake feel is good, and the balance of the car going deeply into corners with trail braking is excellent. Very consistent. With my engine, it is impossible to overheat the brakes, but I suspect I could do it with 300+ hp. There are optional Performance brakes from BMW. They are probably slightly better.
The stock non adaptive suspension is very compliant. It can feel a bit sluggish, but if works well for experienced drivers. In my opinion, the balance is best with a square tyre setup. With the narrower front wheels, you get a bit too much understeer. There is an optional Performance suspension from BMW, but I have never come across any F20 that has it, so I don't know how it performs.
The fake LSD works well for the smaller engines, but I would have gone for a Quaife unit on the M135i.
Another thing to consider, is M4 front control arms for more negative camber. Otherwise you will experience excessive outer tyre wear up front during track days.
The steering feel is a bit vague in the F20 until you push the car to the limit, but when you take it to the limit, you get plenty of feedback, even on ice tracks.
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