As I read tascam product page, this one will gang channels. They are using Texas instruments op amps so I guess no mods are needed.
The HDMI is a feature i was not acquainted with. It looks like some cameras like the Panasonic gh4 will output both time code and start/stop information over HDMI. Presumably this means you can start recording and stop recording on the 701 just by starting a video recording on a gh4, and then the two devices are synced via time code signal from the camera over HDMI cable.
I gather you can also sync multiple 701s via the HDMI port, but that would seem an expensive way of adding more channels given the $599 price tag of the 701.
If you have the right camera and want to sync with the 701, could make sense for that use, but for pure audio recording, I think it will be a hard sale to convince tapers to spend twice as much to get a 701 instead of a 70d.