+1 to all of the others. If your issue is with power, then M3 is the wrong way to go. Put a JB4 on your 135i and you'll have more power than an M3.
Now, there's no question that an M3 will have better general handling/response than a 135i. And if that's what you want, then I think it's a reasonable trade. But if you just want power, the 135i is a better platform than the M3.
2008 135i Montego Blue sport pkg, premium pkg, Logic 7 sound, iDrive/Nav
Mods completed: JB4+ & FBO || LSD || BMW-P suspension + F/R sway bars || 18" Breyton GTS-R || M3 front bushings || Vogtland camber plates || BMW-P rotors + Stoptech pads |||| Acquired, not installed: BMS WW Meth kit || Logic HVI intake || Mishimoto OCC |||| Shopping for: Summer tires