MRC Tuning shows off their 620 horsepower supercharged B7 RS4
Here is a very nicely done video from MRC Tuning highlighting their supercharger for the B7 RS4. Are you in North America and have not heard of MRC tuning? That is likely because they are based out of the UK but videos like this are done precisely for the reason of getting the name out there. The supercharger costs 13,000 British Pounds which converts to roughly $20,000 meaning this supercharger is unlikely to become a popular option in the USA but the more options for the platform the merrier.