a) With the H8000FW connected over firewire, I can send up to 8
channels of audio to the unit, and receive up to 8 channels of audio
back post fx?

You can actually send and receive 16 channels, but routing limitations may restrict the usefulness

b) I can use Firewire on the H8000FW for midi purposes? .

You can. This works better in V5.3 than in previous versions.

c) Can I use VSig with the H8000FW via firewire? (Again, bypassing the need for midi cables)

You can. With the H8000FW connected, its MIDI channels should be selectable in Vsig. There is alas no significant speed increase.