I think I am well qualified to answer this as I just purchased my Mystique about 4 weeks ago and just installed my new headunit last week. From what I read on CEG, the premium amp is not very good, while it may be better than the amps that come in your typical OEM HU it doesn't compare to the amps in a decent aftermarket headunit. I also learned from Crutchfield that there may be another issue with keeping the amp, namely that the voltage levels coming out of your new aftmkt HU my not properly match with the amp degrading the sound. Crutch sells an adaptor, but why bother with that when you can bypass the OEM amp altogether. It is not that hard to do and there is a good thread on here somewhere which tells how to do it. To make life easier though you will need a bypass harness, the one I used was made by American International and was part number FWH-5.
I will say after hearing the before and after sound I am SOOO GLAD I bypassed the factory amp, I will be pulling it next week that is where the Satelite radio will go, unless I can find an nice 4 channel amp small enough to mount in place of the factory amp though that might not be possible.
Hope that helps
