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.
Words to my ears. When I was doing proving grounds, I remember being heartily annoyed by the loss of my meta in particular. I'd much rather have the content get suitably harder but that I face it with my gear all working as I understand it than it stay the same and I lose everything I've grown used to -- set bonuses that change my rage generation, my meta gem, and so on -- because it feels like I should have a special set of gear for PGs or just reforge and regem. With reforging gone and gems less common in Warlords, I think an investigation of this option is definitely welcome. I get why it could be very hard to do, and in some ways boosting the difficulty has down sides in that it means it's not the same content for everybody, but I still like the idea.
Now Celestalon has tweeted that this might not be the case forever.
It could be interesting if the idea was also used in Challenge Modes, but I'm not sure it would work there.