Man, that rub is a fair ways in from the outer edge of the tire. Are you sure the wheels have zero offset? The tire seems to be out a bit more than most I've seen. Is the rear axle assembly stock? Looks like the wheel is negative offset or the axle assembly is wider than stock. But could just be the pic.

You will also have to consider that even with a stiffer or higher arched spring, when you hit a real good bump in the road, such as a severly out of aligned bridge abutment, your car is going to sink down on the springs and with the tire sticking out of the wheel well you'll still get tire rub. Maybe not as often as with the softer spring but in a severe case the spring may drop enough to have the axle tube contact the bump stop. With the tires out like that they will still contacted the fender.

I noticed a tiny line of blue on my tires when I was in Calgary last summer. Right on the outside corner of the tire. I was also getting a vibration when decelerating from about 110 MPH. Couldn't figure out why the tire was contacting the fender while driving around tight curves at high speed. Wasn't due to any large drops or bumps in the road. When I got home I checked everything I could think of and discovered the differential carrier spacers had come loose in the axle housing. Of course the axle shafts had rubbed against the spacers and filled the differential with grindings so I had to take the thing apart to clean it up and reinstall with new spacers and bearings. The differential was moving in the housing and so were the axles which caused the tire rub. That's why I asked if the bearings were tight on your axles. When the retainer and bearing on my GTO axle came adrift the fender cut my tire so badly I had to buy new tires.