I've done the conversion but just saw this thread, sorry.
135i rear hubs are 10mm wider each side than 1M.
This can be sorted with the right combination of hub flanges and rear brake discs or wheels with 10mm more offset. If anyone is interested I'll update this post with the parts required for the mod. Also, my converted hubs will be available soon (to make way for full M3 rear end).
There is a lot of room on the inside so I'd even go more than 10mm extra if planning wide wheels. With the converted hubs I'm running 19x11 ET25 with 295/30 Pilot Super Sports & -2 camber and they rub very slightly on the guard.
