Well since a common upgrade on lesser models is solid polyurethane personally I would not be concerned about 'damage or control issues during use'. High-power S4s (600+hp) can snap the rear diff mount with poly, but that's from repeated hard launches. Never going to happen with a stock V8 and 5hp24. You will get more drivetrain noise and vibration transmitted in to the cabin with solid rubber, but with the benefit of better drivetrain stability. Will you notice either on just the rear diff on an otherwise stock car? Doubtful. They'd be pretty useful for my biturbo project though