Handling vs grip

[Bean]

If I try to interpret your description it seems that with a change in tires the front gains more grip than the rear... with slicks the edge of loosing the grip is very narrow, regualr street tires would have a wide edge. Anyway what you can try is to stiffen upvthe backend a bit to make it more predictable and giving you a better response. If you want more grip in the back do some testing without the rollbar at the rear as this will work against oversteer. I assume you have replaced any worn bushings already. Also your car has probably a striped out rear and no wing, the resulting loss of downforce and thus grip won't help either. Have you had the car on cornerscales once? To test if this is an issue you could chuck some sandbags or other heavy weights in the boot for a run - obviously this is just a test solutiin as you don't want to increase the weight of the car.
Looking at the front less caster and more camber could help towards a lighter steering feel, while drifting.
A difficult question but on a scale of 1-10 how much does you chassis roll while cornering and drifting (10 being completly flat and 1 almost flipping over) - video or pics of the car (taken face on throu a corner) on tarmac would do too.