From what I have heard the OBX system dont hold up well to racing, mostly for street use. The Japs use the KAAZ and CUSICO? for serious drifting both I think are clutch type (not cheap!). The only other gear system that is used for hard core racing is the TORSENs on the MAZDAS and SUBARUs. I am not sure if the problem with disengaging with 1 wheel lift is present in the TORSEN but to my knowledge no one has swapped out the Subies R180s to the nissans (27 splines vs 30 for subies) on the imput shafts. I wish someone would fab up a inexpensive fix for this, the subies are cheaper and easier to get. If anyone does let us know I would be very intrested in converting to the Subie R180s.
I have done my homework and the 06+ STIs come with the R180 TORSENS in the rear diff. and have the 115mm ring gear. They offer a 3.7, 3.9 4.11 R&Ps on diff makes and if you want to run older Nissan 115s you will need to change the mounting bolts (fine vs coarse threads) or retap the nissan ring gear.
A friend who use to run IMSA was telling me that for hard core racing the clutch type is prefered due to the fact the it is adjustable as far as how fast and abrupt you get total lock up, but most racers dont have the tools or expertise to really get use out of the units. How many of us are going to pull your rearend and adjust between sessions or have multiple diffs in varying configs in same ratio (esp useful if raining). Also depending on condition of the discs and springs the units will change there hookup with age/wear, temp and oil viscosity will also have effects on the action. The gear type have the advantage of a more linier action and are not affeced by spring/clutch wear or temp (no adjustability though) also much lower maintance and wear which tranlates to longer life