318is suspension compared to E30 M3?

[b318isp]

However you're onto something, in motorsport the rear axle was available in the angles you quote, however you had to pair the crossmember and trailingarms. 15 degree crossmember had to have 15 drgree trailingarms. And vice versa.

You're right, the changes in angle were on the race models:

http://e30m3performance.com/tech_articles/susp-tech/rear_curves/index.htm