From personal experience i would have to say a bit of 2) & 3)
I've mainly found a small difference between HDMI in Audio quality, not so much with the picture quality ( except with the 1st 3 HDMI's i got with a PS3 from Blockbusters which didnt work as claimed ), if at all perceptable with my dodgey eyes.
However I did find the picture quality improved substantially on my TV for SD programmes with a mains conditioner, even my OH commented on it without knowing what i had done. I cant comment on the HD programmes side, as since the introduction of the conditioners, i now have a V HD box plugged in & not tested to see if i, or the OH can tell the difference.
A mains conditioner did'nt make a scrap of difference to the audio quality in my case, either on my current ( excuse the pun ) or last amp, a Denon 1910, it did however strangle the Pioneer 1017 i had.
Speaker cable, never tested it, but i think it makes a bit of difference.
So as far as i am concerned a bit of a mixed bag.