I think I would go with better shocks that FSD's. Maybe Koni yellows with some mild springs.
The rear M3 subframe bushing made a HUGE difference in the way my car drives. It really firmed up the ride. But not in a bad harsh way. It just got rid of that wiggle from the rear end when going over man hole covers. High speed stability is MUCH improved. It even made teh front suspension feel like its working better now. To me it feels like less body roll and no wiggle at all. But the ride is totally nice - not harsh or bumpy at all.
Maybe this will help you decide...
Dackel's replacement of rear suspension OE bushings with M3 ones…
http://www.1addicts.com/forums/showt...1#post11189793