Many members are concerned about the ride quality of larger rims and sport springs. I owned the stock for a short period of time. A few days for the wheels and 2 months for the springs. Then I swapped to aftermarkets. I almost forgot how the stock ride quality is until I have a chance to drive a 535GT for a few days (don't ask me why, can't disclose). Here is the review:
528i
Forgestar 20" 9J front 10.5J rear
Yokohama Advan Sport tires 255/35/20 front 285/30/20 rear
H&R sport springs
535GT
18" OEM rims 8J front and rear
RFT 245/50/18 front and rear
stock springs front, pneumatic suspension rear
Road comfort: Comparing the two, the 528 is only a bit more harsh on road, over road hump, potholes, speed bumps & etc. May be around 10-15% more harsh I would say. Definitely more noisy for the low profile tires especially on slopes with scored road service. Not sure if the Advan is noisier than other competitive tires such as PS2, Pzero & etc. But my tire guy said they are about the same. I can still feel the road with the 535GT but not as well as the 528. Don't know why but the 535 stock's road feel is actually better than my 528 stock, not sure if is because the GT's wheel is one size bigger than the F10 or I only drove a few days for my 528 RFT therefore the tires had not broken-in.
Cornering: The 535GT amazingly corners pretty well without too much body roll. I suspect is the rear pneumatic suspension. Body roll is just a touch more than my 528 with sport springs. 535GT feels quite tight on twisty road, I don't realize I am driving such a big car. However still not as well as my 528.
Aesthetic: No need to say more.
Conclusion: 20" wheels and sport springs is a touch harsher than the stock but won't ruin your ride comfort. Better performance on cornering, no more puking for your passengers (please note that review not only by me but also another passenger i.e. my wife
). Would I recommend the swap? Definitely yes. However, it's just personal preference at the end of the day. You make the call.