Newbie325cic-
See this thread for thoughts on cabrio colors:
http://www.bimmerfest.com/forums/showthread.php?s=&threadid=12322
My concern with steel grey and black would be that it wouldn't be distinctive. Check the picture of steel grey with red - the contrast of the red brings out the "flavor" in steel grey. Black might have the opposite effect, making the grey more bland.
On the other hand, I've never seen the combo in person so I may be way off base :dunno:
I think you're not getting many responses because that color combo doesn't strike a particular cord with too many people. If you've seen it personally, and loved it, I'd go with it!