Ok, I'll throw my 0.02 in.
The E30 M3 and 325 ARE the same. In my race group there are 4 E30 M3s and 6 E30 325is (and 2 911s, and 1 996
. I have done more engine swaps in the M3s then there are cars, and have built more cages and converted all but 2 of the 325s to SpecE30. Inside all that is different are the seats (and the little dash controls).
AFAIK, Except for springs, shocks, & swaybars (which are luxury car soft on both stock vehicles): The rear end is identical except for LSD, R&P ratio. The front end is the same except for the control arm & rear control arm bushing (The M3 front control arm is aluminum to save unspring weight but has the same geometry.) ALL of which are easily replaced to better then M3 specs. Rear: Offset trailing arm bushings (adj camber & toe) & LSD. Front: offset control arm bushing gets the M3s caster, and a camber plate gets even more camber. Add springs/shocks/sways and it would eat a STOCK M3. Now, the M3 does make marginally more peak power but the 325 romps on it in the torque delivery department. The SpecE30 rear end (3.73) really hurts it though. 5th just isn't useable at the track.
Summary: Anything that can make a M3 fast can be used to make the 325 just as quick (other than the engine). The wheezy, rattley, noisy S14 just makes you THINK your're going faster
SMD (who has both one of the 911s and one of the SpecE30s in the group)