The E34 does feel heavy to drive on the track, I drove, E34, E36, E24 all prepared for track or race duty and I could corner much faster in the lighter E36. The ligther you can get the car will give you a huge advange on the track.
An E28 or E24 feels much lighter and is easier to drive around the track, it is also more tail happy. But there isn't much comparaison between racing an M3 that was designed to hit the track and an M5 that was designed to go get the kids from school fast and in style.
But driving a 350 or 400 HP 635csi turbocharged is a lot of fun... Ask Dave.
Webmaster of http://www.bmwe34.net
'85 Euro 635csi M30 Turbo race car
'92 525i Touring