AFAIK the brand doesn't change the actual GPU in the card at all. Clocks might be different, circuit board design might be different, but it'll look exactly the same device for the computer whether it's Club3D Radeon or a Sapphire Radeon of same number, or whatever brands there are. Clocks might be set differently but what the GPU does,
should by all logic be the same regardless of brand... but as we all know, logic and common sense don't necessarily apply with computers.
The only thing I can think that might make a difference is if different brands use customized BIOSes in their cards, but I dunno how much, if at all, they change the actual workings of the card. But since the drivers are the same for different brands, I would think that isn't an issue.
But again, common sense and computers rarely share a common ground...