It may be worth fitting "Cosworth rear suspension shims", small simple, but they seem to make a difference on my car (Konis & lowered).
http://www.randbmotorsport.co.uk/shop/Parts.asp?ID=5
Speak to MikeR about them.

Also I tried mine set on full hard, fine for track (smooth road) but too responsive and unpredicable, IMHO, soften mine a bit at the rear, and bit more at the front.
+ shims, mine now handles really well IMO.

Gav