George,
Based on your description, it seems your installer did a good job of positioning the rack. If you slide the rack aft to position the fairing behind the antenna base, the light bar will obstruct your sunroof by a good three or four inches. Any further forward and the aft end of the rack's sunroof opening will obstruct.
I installed my rack shortly after getting the truck and I don't remember what XM performance was like then. I know that when I'm near large power lines XM fades. I'd never considered that the Gobi fairing is blocking enough signal to cause XM to fade when it might not normally do so but your suspicion does make sense.
I've considered removing the fairing for re-powder coating. If that happens, I'll be sure and experiment with the XM signal and report back.
Rod
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