i often see people selling 704 suspension in the classifieds. not the greatest but at least you know how it works.
also consider upgrading sway bars and adding strut tower bars if you want to reduce body roll and stiffen the chassis
upgrading suspension performance at stock ride height is rare but i'm sure you can find something if you look hard enough. some bit of lowering is always involved if you want better handling because it lowers the cg and makes the total suspension stroke smaller.
http://f30.bimmerpost.com/forums/sho...b6+acs+springs
i'd say go bilstein factory replacement springs to go with the b6 just to keep it brand matched, idk.
if i were buying struts/shocks/springs combo, i'd rather just get the B14 PSS and accept the lowering even at max height. plus a it'll look better a lil lowered anyway