Cumberbatch could modulate his voice down an octave to make it sound more booming if he needed to -- this is an example of his voice from another character, so he's not trying to sound like Unicron per se. But then again, John Noble who played Denethor in Lord of the Rings was Unicron in TF Prime, and I personally didn't think his Unicron voice was that good (sounds too "cartoony" to me). But he doesn't quite have that baritone/booming quality that Cumberbatch's voice has. <shrugs>