I couldn't find any information about 5.1 and 7.1 support through S/PDIF RCA and/or Toslink on Impact Twin. is that possible?
if you route the signal directly from PC through firewire to SPDIF output, it will be bit transparent - So yes, it is possible.
You cannot really pass through the mixer because that could alter your signal. you still need a decoder and DAC to convert the signal to your 6 or 8 analog channels.