I think I am suffering more or less the same issue with wandering at higher speeds and a general "not as tight as it should be" feeling... I also had mine aligned on a Hunter after having my suspension changed and it was fine for a while but I now suspect pretty much most of the bushes are worn front and rear...
With you replacing your front arms with M3 parts then it is less likely to be the front end though still definitely worth checking around. My guess would be the rear end subframe bushes and possibly the rear arms if not been changed....
Soon I will do every bush on the car, most likely with this kit -
http://www.strongflexuk.co.uk/bmw/3-...-bush-kit.html