I've looked around, and thought the Eaton M45 or M62 would be nice. They are Roots-type positive displacement, or constant pressure. The problem is getting the mounting brackets, which apparently do not exist. Someone on this list with machine shop access was going to let us know when they got them made.
The advantage of the Supercharger is, depending on the model, you can turn it on and off. Also, you have your stock compression 9.2 on the MKIII or 10:1 on the MKIV, when the SC is not in use. Spooltime is minimal, and you eliminate some of the heat concerns associated with turbochargers, although you can put an intercooler on a blower also.