Yes, the 6670 is the highest performing reference design that doesn't need a PCIe plug.
I'm sure you are aware of it, but just throwing it out there for clarity, but time and again there are non-reference designs of higher-performing cards that cut down on power and drop the extra PCIe power requirement. The best example I can remember is the 9800GT "green" edition (I can't be sure now, but I remember it was also low profile aside from not needing external power?). More recently, I believe a non-reference 5750 also followed suit.
Haven't seen any in the 6xxx or corresponding nV series any non-reference card to do such a thing, but who knows, so if you or anyone else is looking for the absolute best performing card without the need for PCIe power, a little window shopping might be a good course of action.