I removed the rear seats in my E46 coupe once for a track day and never did it again. I actually had the opposite problem than the one m_parade experienced. When I took the seats out, the back end felt like it was flexing a lot more and lost a lot of rigidity. As a result, the understeer was quite noticeable. I was pushing through corners a bit more and throttle steering became much less linear.
I assume that since no one has mentioned this, the E92 doesn't have the same problem?