Have you got any photos of the front of the car whilst cornering, it would be good to see how much roll the car is getting.

You say the front is good, but presuming the body isnt twisting too much if the inside rear is getting too light eg. LHR, that means the RHF is falling over too far.Thus the heavier front bar, dont desmiss this unlees your car is understeering(pushing) like a pig.

Another thing that can cause this is insufficient rear end lateral locationing.

As you corner the diff moves sideways under the body puting more load on the outside rear tire and less on the inside. the more grip your tires have the worse it becomes.
Solution: Panhard bar, watts linc or "A" arm.

But if your using stardard road tires this shouldnt be a big problem. without these devices i would use a urethane front spring eye bush.

I would look at these two areas first.

Simon

Edit: all suspension should have bump rubbers wether it be on the body the diff, spring seat or in the shock itself most race cars use the latter. The suspension should not contact these at any time durring normal driving/racing. They are there to prevent sudden shock loadings of metal to metal contact in the event of running into unforseen objects or holes. In your case the curbs around the track! or running off!