Witwer seemed a little crazy in that BTS video. He seems really into just how mentally and physically broken Darth Maul seems to be. Still, it was very cool to see him in the voice over studio working with Clancy Brown. And the incorporation of the Sith Code? That's some serious EU canon to throw into the mix.
Seeing that video though, I don't know if Serafinowicz would have taken it as seriously. He's an incredibly funny guy, and his voiceover work for TPM was great. But he wound up delivering all of what, 3 lines of dialogue in Episode I. It didn't seem like as much of an acting job as much as it was a few punches of ADR. I think he wound up having more dialogue in one of the Episode I trailers than he had in the entire film.