Personally, I'm having more "cut-outs" with the SIR1 than I did with my old FM Sirius adapter... looks like I might have to upgrade to the SIR2. OR, start moving the warts around on the back of the Mustang.
I've also got both XM and Sirius installed although I tend only to listen to Sirius Channel 11 (Radio 1 from the UK)