IMO McDiarmid was amazing in ROTS, note perfect in fact. But it was the terrible, terrible makeup, script, CGI additions to his eyes, CGI body doubles, soft lighting, CGI lighting, editing, plotting, Ligtsaber design, presence of Lightsaber, Lightsaber choreography, pacing, costume, sets etc etc that let him down (Let me know if I left anything out).
The voice had nothing to do with it.*
Go watch the long shots in Palps office at around 01hr18mins, where you can't see how bad the makeup is and it's damn good. You also have to remember that Lucas no longer has the ability to judge a good take from the bad ("You will know!"), all he can see is wether they are reading the lines out as he's written.
(*However, if you listen, Lucas completely f*cked with his voice in post-production too. Massively deepening it at points to near comic levels.)