True, I remember watching the movie back in 2005 and thinking how strange that George Lucas would make a manipulative Sith Lord like him so... wild for a Star Wars movie.
I agree with Greedo - have you ever seen someone who is SO pleased that their plan came to fruition that they are giddy with excitement?
Well, that's how I always viewed that scene with Sidious fighting Yoda.
Think about how long he's wanted to take on his "little green friend" in battle.
In ROTJ, Luke also picked up on it when he told the Emperor that is over-confidence was his weakness.
Clearly, in the battle with Yoda, his over-confidence is getting the better of him, so much so that he does loose his lightsaber and there is a brief glimpse during the fight when they are shooting lightning at each other that he does look like "oh shoot this isn't going as well as I had planned" and then at the last second, he actually wins by blasting Yoda off the ledge.
Even Robot Chicken picked up on this in their skit where Darth Vader calls the Emperor after the first Death Star blows up, Palpatine is talking with two advisors at the beginning and the dialogue picks up with Palpatine saying "So I threw the whole senate at him. The WHOLE SENATE!" and then laughing.
So for me, I think the head sculpt is perfect, now if we could just get a SA ROTS Yoda that he could duel with I'd be happy.