The bias ratio difference only works at very low Mu's, say ice to light snow, then the two ratios have EXACTLY the same transfer slope. (Torsen and SAE papers are widely available).

You can achieve very similar bias increase by just increasing the oil weight (see advice to SAE racers). Save $300!

Remember that changing the diff will put you in an entirely different class for no real gain.

On a technical note, this is the center diff we are talking about(!). So, it really only comes into play with massive torque, or really high slip angles at full throttle. 2:1 R:F is not a problem, IMHO.

In autocross, a solid diff will gain a couple seconds on a minute course (4kq). This is because of the massive and abrupt weight transfers that load and unload each end. I would suspect that the difference on a dry track would be pretty slim even with a full locker. Now in the rain or narrow class spec. tires I would go locker.

I would focus on chassis/handling and make sure that I have enough droop to keep the tires in contact, and stiff enough that weight transfer isn't killing you. Then, the center dif is not really a factor.

I would be a lot more concerned about the damn EDL. (This is why I asked about the Phantom grip diff insert for the rear.) Rear diff bias ratio is a bigger factor than center diff.