I've always been told to add grip where required rather than reduce grip from the other end! How true/practical this is I have no idea. I know suspension is hugely complex and sometimes counter intuitive but the real world experience with Caterhams suggests that to increase grip at the front one or more of the following are required:
More rake (which probably reduces rear grip)
More negative camber
Toe Out My (admittedly somewhat lardy) Duratec car has about 2.75deg negative camber, a fair bit of toe out (1mm I think but not sure what that translates to in angle) and 20mm rake and it turns in brilliantly. It's not too oversteery so I think the balance is pretty good although it can be a handful in the wet. Bear in mind I'm on list 1B tyres so your A539s might behave differently. I should also say that a lot of toe out makes the car a bit twitchy on the road and it follows cambers/bumps a bit too much but you do get used to it.