Thinking he's lying is reasonable, but the hostility comes out of nowhere. His disbelief is adequately covered with the whole "I don't believe that" thing.
I suppose he could be hostile because he's being held captive like that, but then his anger would be much less directed, which it is. Portrayed by his cool and arrogant "air". The "I'll never join you" is too directed at the notion that dooku is inherintly evil. It's like he already knows dooku is a sith. And since we don't have mcgregor on hand to record new lines I suggest the only reasonable way of dealing with it is to get rid of the whole "join me" thing. But it might make the convo seem pointless and even more broken up.
That's the problem with the prequels really. They're too messed up at a too deep level.