In a previous life, I did satellite link budgets for a living. From looking at the Effective Isotropic Radiated Power (EIRP) plot, which measures the transmit power to each area, you can see that Florida is about 59-62 dBW, and Colorado is 65-66 dBW. That typical 5 dB difference in power to the receiver is HUGE for a satellite system near threshold. In fact, I have more to gain in Colorado from the higher elevations that the Sirius satellites give, than Florida, but I have no complaints with XM here. The elevations from Colorado are probably about 10˚ lower than Florida for the XM satellites. Therefore, it seems that the XM design of the satellite antenna pattern just took a little too much away from south Texas, the Gulf Coast, and Florida, making the incorrect assumption that the better elevation angles there would compensate the weaker signal.
Remember that the Sirius signal structure and receiver are different than the XM signal structure and receiver, even though they are one company now. Unless Telsa has a dual Sirius/XM receiver in the Model S, changing from Sirius to XM will take different, replacement hardware.