Quick specs search shows a 2001 530i with a 3L I6 makes ~225 with a curb weight of about 3350.
That's weight/power of 14.8 vs. 99 boxster's 14.0. Drag coef makes it a toss up, and if yer top was down I think the Bimmer wins. Chances are if he played with you, he may have done something to his car, too. It's very close, and barring any major factors, I don't see why you should expect to beat him easily.
If you wanted to make a point of it, you shoulda baited him into a twisty road
-David