I say ... GO FOR IT!
The 318is I just bought has Bilsteins and Tokico lowering springs, which are a rough equivalent to H&R sport springs.
The car rides and handles wonderfully. It is not harsh. The ride height and "stance" (I hate that trendy term, but it's useful) is perfect.
My frame of reference is a little screwy however, because I've been daily driving an STX-prepped autoX e30 that has race dampers and much higher spring rates.
d.hitchcock
I say go for the Billies and H&Rs unless you plan to autocross, in which case the setup will not function as well.