I have about 200 rounds through my 6 GT upper. Thus far I've had a few rounds where the polymer tip gets knocked off factory 109 ELDM during feeding and I haven't determined exactly what's causing it. I've polished the feed ramps and made sure there are no sharp edges in the barrel extension. It runs better since the ramps were polished, but I've still experienced a damaged polymer tips since that point.
I'll be using it in match conditions for the first time this coming weekend.
As for 6mm ARC, there is brass available. Hornady has finally released brass and there is no shortage of Hornady ammo on the market. Starline has released brass 3 times in the last month or so. I've shot 3 PRS matches so far this season with 6 ARC and really like it.