I know for a fact that Kenny Duttweiler did dyno testing with the BGC intake so he would probably be the best source of info on if there were any gains over a stock ported unit....
I have one on my stock block....ran a 10:16 with it. It was on my old stage block originally.
I never did any sort of stock manifold/BGC manifold exchange to get proof so I can't help there.
I don't like to put words in someone else's mouth since my fuzzy little brain doesn't remember crap like it used to, but I believe Jason Cramer said it was the best flowing manifold he's done. Or something like that. Perhaps he said it was the best AFTER his power plate...dunno for certain. But for a real world answer you should phone Jason. He's also easier to get ahold of than Dutweiler.
Jason probably said that it was the best AFTER his power plate. I am sure he had the best gains with this intake since the cylinder to cylinder variations on the BGC are (IMO) worse than stock. I am coming up on a year of trouble free passes since I put his power plate on. I had NEVER gone that long once the car got in the 8's.