I went last year in my Z4, and plan to visit this year during July in the GP.
At least one GP has visited the Ring already.
Official (?) records:
A complete lap (BTG + some):
MCS Works 200 bhp 8:55 min
A complete lap (BTG + some): GP 218 bhp 8:41 min.
I hope S&M will organize a new Ring event in 2007 *hint, hint*