First, you are not going to get any more performance from 18's than from the 17's. I have never had an E39 wheel on my E34's or any other wheel for that matter requiring hubcentric spacers that I did not have some issue with. Mainly vibration and shimmy at some point between 70 to 100. Since I rarely see anything north of 100, I cannot comment on the higher speeds.
The 17" Throwing Star wheel, as you should well know, is the best wheel ever made for an E34 and the lightest and strongest and best looking and, and, and.
Smile, be happy, you are one of the few who have the best wheel out there for their E34.
Just my .02
Mike Holbrook
Meridian, Idaho
1992 535im, 17", Euro M5 Throwing Stars 8's & 9's, FK-451 235/45s & 255/40s, M5 Sway Bars 25/20, Conforti chip, Lowes Ram Air, glass sunroof