First off, to Dave's comment, all the mis-information came from my post. I don't see any issues with the rest of the thread. It is pretty bad that I run a MazdaComp and have no idea what type of diff it is. I did go back and do some research on it after Bench set me straight.
Although my credibility in regards to diffs is probably shot as this point, I will comment anyway. I may not be up to speed on clutch or viscous type diffs, but I know quite a bit about Torsens, both type 1 and type 2. I have disassembled examples of both and designed and machined custom housings for them for use in formula SAE cars. Robert is correct in that they are completely mechancial, i.e. it has no clutches. What they do have are thrust washers between the housing and the sun gears. I have heard people refer to these as clutches, but they are in fact just smooth metal washers. This is true for both type 1 and type 2. The difference between the two diffs is that the planetary gears on the type 2 rotate along the same axis as the sun gears. This was done, at least in my opinion, to reduce maching and overall part costs. The majority of the Torsen's torque biasing comes from the gear set and the friction between the gears, however, the friction between the sun gear and the housing can change the torque bias of the diff. to an extent. I have seen people run thrust washers of varying materials, including carbon fiber, in order to tune the torque bias of the diff. I have also heard of people replacing the thrust washers with needle bearings to take the torque bias the other way.
A few other interesting points on the Torsens. The torque bias drops after the diff breaks in. Again, the Torsens rely on friction. Friction goes down after the diff breaks in. Second, in general (may not be the case for the Torsens in the Miatas), the Torsen type 2s have a slightly lower torque bias than the type 1s.
All that said, I would imagine that running the slippery shockproof oil in the Torsens would most likley reduce the torque bias of the diff, given that it would reduce the friction between the gears, and between the sun gear/housing and the thrust washer. The shockproof oil is however lighter weight (al least the lightweight and super leightweight stuff) than 75-90. You may be able to make up for the loss in torque bias with the power increase from running the lighter fluid? I would imagine this would be very track dependant.
I have not felt (subjectively) any performance loss with my MazdaComp diff running the shockproof oil, nor have I seen any change in lap times. I will continue to run it. As I stated earlier, with the small ring and pinion, I am more worried about keeping it alive than I am of any torque bias loss.