Well, it's common knowledge that the 6 pin connectors that are specced for 75W can safely deliver 150W+. People use 6->8 pin adapters all the time and as you already said, the 6+2 pin connectors only add 2 more ground wires. However, if you're connecting a power hungry card like a 5970 to a single line, you're going to be drawing up to 225W from that (more if overclocked). I'm not sure if that's still within safe limits (the wires probably won't overheat, but the connector could get damaged at the point where you're splitting it).
If you still want to risk it, you should also check if your PSU can deliver that much on a single rail.