Hey, as title says, my friend has msi 960 2gb and i have sapphire nitro+ 480 4gb we tried to compare fps and he has more, and also in gta when i go too fast sometimes sky flashes black, got my gpu last friday...
Check your driver and OS install, and make sure your components aren't overheating and massively throttling back. Your build SHOULD be crushing, but give us more details.