We will be running a custom made rear anti-roll bar on our Cinq track car. Once its complete I will post up pictures but its made from a 19mm front MGF anti roll bar, cut, sleeved welded and so far mocked up with Lancia integrale EVO front rose jointed drop links.
There was a rear anti-roll bar on Adz old Cinq, made from I think a VX Cartlon rear bar, it worked well as the lifting of inside wheel and much flatter corning than you expect from Cinqs with even stiff suspension as pictures of it going round Castle Combe show clearly.
On the front bar I would remove and check the bushes they will be getting old now, on one we removed from our Cinq they were long past there best, badly compressed and cracked and perishing. I'd replace these with minimum new ones or get some universal poly ones, the bar is approx 18.5mm diameter IIRC.
Having fitted a WhiteLineFlatOut rear anti-roll bar to my Suzuki Ignis Sport which had none as standard the turn in was incredible, made it have near no understeer, it also ran a uprated matched WhilteLine front, though many owners ran standard front and the WhiteLine rear and loved it.
I also run a WhilteLine adjustable rear anti-roll bar on my GT4 which has eliminated the understeer, in fact on hardest setting makes rear end very tail happy, fun in a 4WD car.
You can if running a rear anti-roll bar reduce spring rates and get a more compliant ride as a result, but with better turn in and less roll.
As has been explained some owners have removed the front anti-roll bar and then compensated by running much harder springs to resist roll.
We have also modified our rear arms to give some negative camber, the tolerance from factory is pretty big, eye opening big in fact though don't have the figures to hand we are aiming for 2degrees -ve camber rear.
Remember not to confuse stiffness for overall grip. having driving Knockhill in a variety of machines its a bumpy track with many elevation changes that favor some compliance in the suspension.
Forgot to add, driver skill plays huge part in it, in Jamie's near standard Clio 172 Phase 2 on very cheap 3 diff makes of tyres, 2 matching Nexens front, one Kumho and Champiro??? fitted rear as fitted to track wheels picked up from ebay for £85. At Castle Combe on Fri he went past a Corsa running a redtop with ease, and the stripped out Clio 172 phase 1 on Dunlop slicks, LSD and high lift cams was barely quicker, and unfortunetly that car ended up in barriers later on the day. Remarkably those tyres did something like 40 laps of CC which is regarded as one of the quickest club circuits in the UK.
) or do you like it to all feel quite tight and suddenly let go on you (that all sounds dodgy 
