I agree with the initial scenario! From my point of view, it seems the only major advantage of the Io/HD doing the compression onboard is on capture (enough compression to send an HD signal down the FW800). Otherwise I think there may be a disadvantage since your CPU(s) must recompress to pro-res before sending it back down the FW800 to the Io/HD for output. This may indeed cause less RT playback than a PCI-E card. – FWIW