Dennett states, without presenting a single number, that the bandwidth needs for reproducing our sensory experience would be so great that it is impossible (his actual word); and that this proves that we are not brains in vats.

Maybe I was being too generous when I read this chapter, but I don't think that's what Dennett was saying. He was saying that in order for a brain-in-a-vat to work, the operator would have to anticipate every possible observation you could make, resulting in a combinatorial explosion that could not be handled by anything simpler than the universe itself.

That ties in with his next point (that you mention) about hallucinations, and how they persist only until you make an observation that the hallucination-generator can't fake.

It wasn't about bandwidth (rate of information transfer) at all. But perhaps I should re-read it.