If it's a dedicated track car i'd be looking at about 250 - 300 lb" effective, at the rear for something the weight of a GTV!! Then you want front & rear anti-roll bars on top of that...
If you're not seeing any problems braking, or with skittishness, i'd stiffen the front rather than soften the rear. Do you have bushes in your front ARB mountings or links? IF you rose-joint the pick-up's and mount points, this will effectively increase anti-roll and reduce oversteer.
Another option is to play with the ARB pick-up point. If you can knock up a clamp-on adapter so the pick-up is - say 1" further from the end, you're reducing the leverage and so increasing the effort required to twist the bar and hence the effective stiffness.
What you also might want to look out for is the suspension bottoming out on the rubbers. A circuit car only ever really wants to be on the stops when it hits curbing, otherwise the rapid stiffening will induce massive weight transfer and instantaneous understeer or oversteer. If your suspension is modified & lowered, you need to fit shorter & stiffer bump stops.
Drawing board time...
If something simple such as i've mentioned above doesn't work, don't waste your time on anything else. You need to look deeper. Find out what the rear spring rate is, then remove the springs from the front struts and compression test to find what rate these are.
Cut the spring pans off and get some no-weld slip-on overcoil adapters for your front dampers. These have a lip that sits on the top of the damper body with a threaded side which allows you to fit 2 1/4" overcoils. The springs are rather cheap and you could then experiment with different rates and have the advantage of being able to set your corner weights correctly.
Go forth and investigate!!!