E30: stiffest rear springs with Koni SAs?

I have problem with my new slicks where under hard cornering, the tires rub on the inside in the rear. I can't space them out any further, plus I'd rather stiffen up the arse. I have the GC coilover kit, which is not a true coilover in the rear. I currently have 750lb springs there. What are the stiffest I could go with without overwhelming the shocks?

I've been told by GC that I can run about a max 600 lb spring front, which is a true coil over. So I'm guessing this means I may be able to run like a 900-1000 rear?

Some visual aid for non-BMW guys to see the relationship of the spring perch to the shock pickup (top right) and the pivot:

You could call Koni. They would at least know what the shocks are able to handle in a coilover situation. Then to figure out the spring rate needed take off the spring. Then measure hor far the shock moves through it's range of motion. Then figure out how far the spring seat moves through that same range. Do a little math and you e got your answer.

With an E30, you hit a point right near 800 or so where adding more spring isn't really doing anything but flexing the chassis and control arm bushings more (think of it as a pliers...). They handle pretty well right there also... to solve your tire rubbing issue is another matter... one usually solved with a different offset, spacers and Ireland's flares. (or if you are me... a big berkeleying hammer and some paint).

Thanks guys. I've got a half cage with the rear bars tied in roughly to the spring perch locations so I'm hoping the rear end is stiffened up some. The bushings are all urethane. I will be adding fender flares next year with a change in the suspension and wheels/brakes so I'd rather do something that can be used later. GC just replied to me saying that the rear wheel rate is about half and the Konis can take about 650 pounds so 900 in the rear is fine... so I'll try that.

I'll also see if I can introduce Capt Sledge to the rubbed areas for a quickie.