Quaife, Wavetrac and MFactory all have options for the e90/E92.
Swopping with an M3 is a good option, but propshaft needs to be shortened and balanced. So a good engineering contact is a must.
If we can get enough people together, maybe a group buy could help bring costs down. Quaife seems the best bang for buck.
I think the 335i in stock form will benefit from this upgrade.