I am wondering if the e12's performance might be underestimated? Autocar magazine tested both cars and the e28 M5 was only 8 mph faster. The magazine also noted the e12's dogleg 5 speed really hindered the 0-60 mph time with the slow 1 to 2 shift.
e12 M535i at Zolder
Autocar's non a/c plain spec e12 M535i was only 2,971 lbs, whereas many M5s come loaded with powered toys. So maybe there are some aspects of performance where the light, torque biased e12 can shine?
e12 & e28