I've always believed (based solely on what was shown on screen, mind you) that it was Vader's suit that absorbed Han's blaster shots, not Force ability. This is because Luke gets a good solid strike on him with his lightsaber during their duel, which has no effect, save the sparks and evidently startling his father. I can explain that only by postulating that Vader's suit is armored. The presence of armor gives the means to absorb the blaster shots, and thereby reduces it to a mere parlor trick, but one which in conjunction with the telekinesis could be, and was, quite disconcerting. That's a really powerful scene when Han shoots at Vader. (Note also that the script makes the point of telling us back on Hoth that there is armor in the SW universe that is "too strong for blasters".)
Clearly, TESB shows that Vader's telekinetic abilities are very advanced.
I think the OP has a point, though, in that there is a gap between General Grievous's construction and Darth Vader's. Nothing on screen is presented to explain anything but the superficial workings of either being as a cybernetic organism, and so nothing is presented which can explain why Vader isn't and never comes to be as agile as Grievous.
To me, the most compelling explanation for why Vader hasn't received further cybernetic enhancement is that it's for the same reason why Vader does the bidding of the Emperor in the first place. The Emperor has broken his spirit and his will to resist. While he remains under the Emperor's spell, Vader will be exactly what the Emperor wants him to be.
I'm sure if Lucas had made the OT today, Vader's cybernetics would have made him be able to move faster than the eye could see and fly around on boot-jets
Yeah, the PT is really so much
moar than the OT, isn't it.