Then if the uncompressed program running had consciousness and the compressed program running did not, you have either proved or defined consciousness as something which is not an output. If it is possible to do what you are suggesting then consciousness has no effect on behavior, which is the presumption one must make in order to conclude that p-zombies are possible.

I haven't thought about this stuff for a while and my memory is a bit hazy in relation to it so I could be getting things wrong here but this comment doesn't seem right to me.

First, my p-zombie is not just a duplicate of me in terms of my input-output profile. Rather, it's a perfect physical duplicate of me. So one can deny the possibility of zombies while still holding that a computer with the same input-output profile as me is not conscious. For example, one could hold that only carbon-based life could be conscious while denying the possibility of zombies (denying that a physical duplicate of a carbon-based lifeform that is conscious could lack consciousness) while denying that an identical input-output profile implies consciousness.

Second, if it could be shown that the same input-output profile could exist even with consciousness was removed this doesn't show that consciousness can't play a causal role in guiding behaviour. Rather, it shows that the same input-output profile can exist without consciousness. That doesn't mean that consciousness can't cause that input-output profile in one system and something else cause it in the other system.

Third, it seems that one can deny the possibility of zombies while accepting that consciousness has no causal impact on behaviour (contra the last sentence of the quoted fragment): one could hold that the behaviour causes the conscious experience (or that the thing which causes the behaviour also causes the conscious experience). One could then deny that something could be physically identical to me but lack consciousness (that is, deny the possibility of zombies) while still accepting that consciousness lacks causal influence on behaviour.

Am I confused here or do the three points above seem to hold?