Look at post #17 in this thread for some input on BMWP spring rates:
https://www.1addicts.com/forums/show...4#post22187244
It's really nice to have people on here that go thru the trouble of digging into showing this sort of detail.
Unlike with a one size fits all spring, with the BMWP springs you get a relatively consistent drop because they are spec'd for you car's weight. Negative is you'll pay a good bit more for them.
I don't think there is any B6 v B8 type distinction with Koni's. Seems the difference is adjustability and firmness, but no length of spring. Get them dialed in with the correct bumpstops. I think my front bumpstops are from a Z8, rears from an EU car. If you use Tischer (which I recommend) they will get you the correct bumpstops.