Just to give you some idea, I run 171lb Eibach coil-over springs on the rear of mine (with LEDAs ), and I can't go above position 7 of 22 without the back-end hopping on tight 2nd gear corners (which means it is too hard). However, it does depend on you dampers, as the bump / rebound rates along with the spring length all play an important part in spring poundage.

I would say 150lb would be suitable, but it depends on your dampers.

500lb on the rear is way, way too hard IMO (unless you have removed the ARB?), but you will find out soon enough .

Why do people think that hard springs = better handling - the wheel has to stay in contact with the road for it to provide grip .