@BMWlady here is a thread of a 135i that was bagged in the US.
You could probably PM him and get his thoughts on his setup. Bags wouldn't be the best on the track, but I think they would still be better than the stock suspension on these cars. As long as you add the supporting modifications.
Ridetech has been doing air bag suspension on muscle cars for a while now, and they have good results on the autocross tracks. The setup won't be near as good as a set of $8k coilovers though.