There are two separate issues with Sirius XM performance...
Acquiring Signal: Losing the signal in areas where you wouldn't expect to. There are no terrain features present to block the signal... hills, buildings, bridge overpasses, etc. That issue is corrected with the antenna TSB.
XM Service Loading, Please Wait: Random, without warning, dropouts that will take Sirius offline while attempting to reload or reacquire a connection to the service itself. This will mute sound and take down all of your channel presets and other controls related to Sirius for a minute or so until connection is reacquired.
I had both of these issues on mine. The antenna TSB solved the acquiring signal issue with nothing further needing to be done.
My XM service loading issue was a different matter. The dealer updated my U-Connect software and worked with the Sirius sat techs to go through all of the resets and resubscribing of my radio ID. They also ordered a new 8.4AN radio for me.
Funny thing was in the days I waited for the radio I never once experienced the reloading issue, which was a daily occurrence. Never experienced it again after the radio replacement. So I can't say what procedure actually solved that problem.