Alex on my HPC I have changed the front springs to 270lb. This makes a lot of difference when used with adjustable platforms-does your car have these? The front anti-roll bar sounds as though it is too stiff,this will make a drastic improvement. Do you know the ride height on your car? have you had the corner weights set? I had mine done a while ago and can tell you the ride heights if required. What are your camber and castor set at? This can affect it as well. Try some of these things before adjusting the rear anti roll bar. My 7 has not got a rear arb and does not seem to under-steer on the road it feels very nuetral/controlable oversteer which looks really good on the exit of a roundabout!
Good luck :-James