hi mate
after reading your problems again, im a tad confused
longer studs? this would make the spacer even less likely to sit flush am i correct?
you wont be able to eliminate this problem of the studs hitting the rear of your wheels with 15mm of material thickness, there would be a small amount ( of the hub studs ) sticking out through the spacer, which i guess the only way to get rid of this would be to take a small amount off the ends of the studs, which you probably wont want to do
it was the 20mm eibachs i had the trouble with on my rx7 with the ssr's due to the nut design of the spacers, and i had no issues at all with these ones
il ask again tomorrow for you, but with 15mm spacers we might have hit a problem