As a follow up for future readers - I did eventually try the metal version of the 74.1mm to 72.56mm hub rings from http://www.1010tires.com/hubrings.asp as MC suggested earlier in this thread and they made an ABSOLUTELY DRAMATIC difference in the quality of the ride.

In my situation, the chinese aftermarket manufacturer (surprise) labeled their rims as being 72.56mm on the shipping container but manufactured them to 74.1mm probably to fit a wider-variety of BMW's that also use the 120mm bolt pattern.

If your 20" aftermarket wheels just wont ride correctly, get a digital caliper and measure the diameter of the hole to ensure a precision 72.56mm opening. If its larger, get the appropriate rings to compensate for the difference otherwise your new rims will NEVER ride correctly regardless of how many times you have them rebalanced.