I have a track journal that you can see the last pages.
Before I put aero, I always struggled with rear grip. My car was fully stripped too, with solid spherical bushings, and similar tires.
I tried those rates you have and with rcomp, I could barely keep my car on the track.
The z4m chassi is very stiff and with a short wheel base, I think the z4m is better with softer Springs. Here you can see all I've done
My track day journal. new update 11/20/2017
http://www.zpost.com/forums/showthread.php?t=1234284
This is the video when I tried 700/850lbs.
Removed them right away
Today with aero I gave 700/650 lbs with front bar in medium and oem rear bar.
Before aero ended up with 500/550 I think.