Upload & Sell: Off
Jon, has a firmware upgrade ever impacted a third-party grip to your knowledge? I hadn't thought about that possibility.
JDL: No, not to my knowledge, but anything's possible. And it seems a reasonable concern, given the analogous issue with 3rd party lenses.
For me, better safe than sorry, given that Canon has announced a firmware update for the 5DIII coming in April, further updates after that are as likely as not, and I for one intend to keep this body for several years. (I've had my 5DI for 5½ years now, and my 20D for more than 8. There were at least four firmware updates to the 5DI, ending with v1.1.1.)
Over time, the price of the Canon BG-E11 battery grip will likely come down in price. While I would personally stick with the Canon grip, I would be less hesitant to mount a 3rd party grip to my 5DIII if it were manufactured some months after the final 5DIII firmware update.
Good luck with your refund / replacement! Most of the on-line vendors offer extended return periods following the recent holidays.