I have XM and an F800 as well.... Where do you live? I am in Canada, and mine cuts out a fair bit, not because of F800 interference, but because XM signal sucks bad... Had the Sirius system a while ago, and it was great, because their satellites orbit in a figure 8 over north america.... XM have geostationary satellites over the equator, so you will likely always have the same deadspots on your drive..
And before anyone says well, its not either now, because they merged into SiriusXM, the radios must still be on either the Sirius satellites or the XM ones.