Wait a minute
If you've got the right tool, why is it taking you 3-4 hours to change out a set of springs? It shouldn't take more than an hour, especially if you've got the experience of 2 or 3 a week. Even another poster earlier claimed he could do it in under and hour, WITHOUT a spring compressor.
Using the proper compressor I've personally seen a set of springs on a w210 changed in under 30 minutes. 3-4 hours doesn't quite make sense.