Try revalving first.
I went to a Bilstein tech session hosted at their west coast headquarters for the San Diego BMWCCA chapter. They'll revalve your shocks to any spec you'd like for about $55 per corner. A friend of mine is installing brand new Bilsteins in his M roadster, and he has already gone to custom valve settings. If you can hold off a couple of weeks, he'll have them installed with his stock springs and he'll exercise them at a BMWCCA autox on September 7th. I can get back to you with his opinion of the setup.
If you're in a hurry, you can call Bilstein and discuss your spring rates with them for a recommendation. Also be sure to inquire about the benefits of their shock vs Koni's. There are compelling technical reasons to stick with Bilstein.
Mike