I believe you'll find that roll steer will perform in a largely similar fashion to 4WS in high speed corners, i.e. it's evil; and in any case where you're not generating maximum lateral g, you'll have a car which doesn't really turn at all.
You could perhaps reverse the direction of the roll steer, start with some toe out which makes your turn-in plenty good, but then recovers when you get some body roll going. I may test that on our car, actually...
The RX-7 system mentioned earlier on strikes me as being much the same thing, a g-sensitive toe control.