Remember the story Palpatine told Anakin about ye olde Sith Lord who could manipulate the *cringe* midiclorians. In the face of this story it was pretty clear to me that Sidious must be as old as Yoda if not older and that what was revealed was indeed his true form.

In my book he could not keep up concentrating on his masquerade while using his force lightning (i.e. being overwhelmed by his hatred which he had to hold back and hide for such a long time), while at the same time he was of course trying to depict that mace windu was being responsible for this effect taking place.

When it comes down to the doodeling changy voice and the crappy makeup, I'd rather label that sloppy treatment of the scene which was approved by George Lucas with a "good enough".