Regarding Tarkin being more powerful and ruthless, I think that might just be because we see him under different circumstances. In ANH he was in charge of the Death Star, and had little to do other than order Leia's execution and say 'You may fire when ready.'
I can fully imagine that under different circumstances he could be as presented here. Certainly I can hear Peter Cushing deliver most of his dialogue as written here (with the possible exception of the line 'Oh do shut up Solo.' To me, Tarkin would be more grandiose, 'Be silent Solo' for example. But that's a minor quibble).