So what you stating is that Sat signals have FEC and therefore wouldn't be interfered with by Transverse Mode Noise from Dirty AC grid?
Well, I'm not sure about what kind of effects an unclean AC line would have on the receiver, but my guess is that the picture will either work, skip, or not work at all. Yes I have read that the satellite signals use FEC. I would think Transverse Mode Noise would affect the attenuation of the signal, but not enough to make the picture quality suffer. Basically I would look at my point dish screen. FEC is also really good with impulse noise, so I would think even electrical line impulse noise, as well as interference such as rain droplets, and foliage/obstacles will not affect the picture to a certain extent.
Basically, with the error-correction protocols, I believe that as long as the signal bar shows green you will have the same picture quality regardless. I believe that the electrical noise would have more of an effect on reliability than quality. Which could in turn affect how robust it is during rain fade events, just like a poorly peaked dish would, but maybe even to a lesser extent.
However, I am no electronic engineer, so for all I know my theories could be wrong, but they make sense to me.