higher tyre pressure on the BACK will help reduce understeer but only to a point
I run A048 yokohamas and will run them at 25psi HOT all round and it is ideal
it's hard to say what needs done to the car without having details of your current alignment and suspension setup - not what suspension you have but spring preload / suspension height/weight distribution
for circuit use you want to set up the rear of the car so it is neutral and then work from there. some will say whack a huge ARB on the back etc but you don't want to be steering the car from the rear, on a circuit car you want to have a nice planted rear end and once you acheive this then adjust the rest of the suspension to suit. countersteering the car during or whilst exiting a corner looses you time
rear of the car you should not run heaps of toe in, that will do nothing for reducing understeer!!! before using a cage i ran toe out on the back to promote some turn in, after i fitted the cage i backed all this off and run zero toe with -1 deg camber. rear end is PLANTED which goes to show how well this works as i run 20k rear springs which on paper you think the back end would be twitchy as hell
rule of thumb you should always run more front camber (duh) but how much depends on the spring rates and tyres etc
for me i run zero toe up front as i find it gives the best feel from the car, toe out increases steering response but numbs feel somewhat - steering just feels to light
raise the rear end a touch or lower the front a touch, my car is built for circuit use and i always run a touch of rake forward then have a play about with the suspension. i personally would start with zero toe all round and see how you get on
a diff will not help reduce understeer, a diff will only help you with traction on exit of the corner, understeer is the suspension setup
