switching to non-runflats significantly reduces the weight of the wheel-tire combination. With the reduction in unsprung weight and rotating intertia, the car doesn't have choice but to perform better.
This only benefits handling... further, the super stiff sidewall of RFT tires reduces the vehicle stability when traveling through corners with uneven pavement and bumps.
Yes the E90 handles well with RFTs out of the box, but put 30000 km on a set and let me know how you feel about RFTs once they've worn a bit.
I haven't switched to non-RFTs yet, but am planning on doing so.
As was said before, BMW tamed down the ill effects of the RFT tires... ill effects still remain despite their efforts. It is impossible for the cars handling to suffer by switching to non-RFT tires of OEM quality. It is the laws of physics!