You can get irmsher bushes, which were fitted to the Calibra DTM models, becasue they were lowered as standard (about -25m). These are, I belive, £80 from a dealer (pair). Might be able to get them cheaper though autovaux. There is a post somwhere thats quotes the GM P/N for it.
"Q - My car is lowered, how do i correct the rear wheel camber?
A - Ideally with an adjustable camber kit from either Richard King or Regal. However, Irmscher bushes are available for the DTM Calibra (part number - i 2704004) which will correct the camber induced by approximately 25mm drops and are less than half the price of the adjustable camber kits."