The 2 Deg. neg camber you are using, is in the ballpark. When you come off the track, the tires should still be coned. Try increasing camber rise (active camber) by putting a spacer between the upper link and rear hub carrier. Try 3mm.
Max straight line traction would be obtained with 'flat' tires. Due to the lack of differentials, we must run coned tires to get consistant/predictable handling. A coned/tapered tire will be 'scrubbing' at all times(because of the difference in roll-out from the smaller to larger diameter). Un-tapered tire will NOT be scrubbing when going straight. When you turn, the tires fight each other & tend to resist intial turn-in followed by an abrupt loss of side bite.
Coned tires will allow the car to turn-in without hesitation. Other aspects of set-up keep things under control ie. overdrive, shock set-up, sway-bars, etc.
Differentials would allow us to run un-tapered tires and have less tire wear. Would the increased weight, mechanical complexety be worth it? Don't know. I don't know how much extra tire life we could expect.
Most importantly, would it slow down our lap times?
Perhaps, one day, I will find out. Right now my car is rippin' the way it is! Not much incentive to experiment.
Perhaps more info than you wanted.... Try adding the spacer.