Right now, the way Proving Grounds work is simple - you go in, and your gear is scaled down to ilevel 463, with certain specific gear abilities like set bonuses, the legendary sha-touched gems, legendary meta-gems or the legendary cape proc not functional. If you have any of those items, it is as if you do not -- in some ways (such as your meta gem) it's worse than if you were using a lesser meta in an actual ilevel 463 helm, because you're literally getting nothing at all out of your meta.
Now Celestalon has tweeted that this might not be the case forever.
@InValensName We're investigating the potential of scaling PGs up to your ilvl. Not a guarantee, just an investigation. Has Pros/Cons.- Celestalon (@Celestalon) February 24, 2014
It could be interesting if the idea was also used in Challenge Modes, but I'm not sure it would work there.