As much as I appreciate BMW's idea of making the 135i a proper M car, you can tell it's
A) clearly in the shadow of its big brother the E92 (which already was fairly outdated around the time it was released) and
B) basically the best you could do with the chassis of a 7 year old car that wasn't even meant for track performance.
The comparison with the Audi TT RS proves my point pretty well. It may be a GT oriented roadster with loads of understeer, but despite this it's able to stay surprisingly close to the laptimes of the 1M. I believe the M2 Competition and the M2 CS are the definitive way to purchase a performance car below the M3