Having autocrossed my 500E a few times, and having run it on both Lime Rock & Pocono Raceways, I think I've gained a fair amount of understanding of a 500E's handling traits. And I think the rear roll bar's fine! My car's plenty stiff in the rear, at least as stiff as I'd ever want a road car. I think the front's set up right too, although many feel that the front's the end that can be stiffed up a bit. I simply don't feel that way. I think the suspension setup, front and rear, is a very effective compromise between roadability and track characteristics.
Sorry to hear you've heard some rubbing...kind of surprising. What's the offset on those huge wheels???
As to the suspension stuff, just my opinion