DE, I agree that McDiarimids voice performance in ESB was flat. but it is not all a total loss. if you combine emperor dialogue from ROTJ and ROTS its probably salvageable and certainly something I'll be attempting on my version.
in addition, I think Ian McD's scene in ESB stands out because we see too much of his face.

in ROTJ he's in a lot more shadow, particularly under the chin.

so a bit of masking, some interferenec and chroma keying etc to blend it in, I think it would be possible to darken him up and retain the scene visually and make it fit better with ROTJ.