there are 9 ground pins on a x1 connector, and it goes to the mobos ground plane. ground is ground at that point (and its a huge, relatively speaking, amount of copper) and its shared among all PSU ground wires that directly connect to the mobo. count em up; there are a lot.
the original burning problem was (in almost all cases) the limited number +12v power connectors to the PCIe bus via the AUX connector. not the ground path.
Actually you are probably right, i did not check the ground connector count *whups*
Well, in any case, it's a good idea to also include extra grounding, just for that extra safety margin