That whole thing about Vader being less agile because of the cybernetics rather falls over when you look at Grievous and how insanely agile he was with *his* cybernetics. Hell, he had a Robocop style full body prosthesis!
I do vaguely recall Lucas saying something to the effect of his reduced biomass (because, sans limbs) is the cause because it means fewer midichlorians...
So much for "size matters not", no?
I suppose it's possible that Palpy is doing something to limit and control Vader. Perhaps he's syphoning off his power to keep himself going?