So I have the 2gb c720. I just shut most everything down except for a couple of terminal windows that didn't look like they had anything going on. I am on Gnome Shell on 14.04, and with the compositing perhaps this makes this crappier. I also had the device in the USB 3 port.

I was able to get 6 megs a sec with GQRX. 7 had occasional but reasonable stuttering. It is possible that without the compositing this would be better, and also I would think a GRC graph without any UI updating (or even without X) may perform even better.