$4k sounds a bit much for a differential. The stock m3 lsd diff is under $2k, and it's a clutch type.
Originally Posted by vasillalov
One other thing to consider: The cheapest is not always the best.
Quaife is very popular but it is a torque sensing diff. It has its disadvantages: it requires that both tires have some amount of friction for it to lock. If one tire lifts off the ground or it slips on ice, oil, etc then there quaife acts like an open diff. Wavetrack claims that they provide locking even if one tire has no traction.
BOTH quaife and wavetrack require that there is torque coming out of the drive shaft in order to lock. This means that they only lock when you are accelerating. If you are decelerating, they both act as open diffs!
Drexler, OS Giken and some others make clutch type differentials which exhibit none of those disadvantages. They are more expensive though.
My recommendation: talk to Harold@HPA. The guy knows what he is doing and he is definitely the man to get in touch about anything related to drivetrain, suspensions, brakes...
EDIT: I will be dropping close to $4K for an LSD after the holidays! My business is going to Harold.