Well, to be honest, if you're playing any game above 60FPS, you won't notice any difference between 60 and 300. Thats just due to the nature of the human eye. I would say the 300 FPS is because the demo you're looking at is probably pretty old (note, I have no idea what demo you're talking about). I would just accept it and be glad your card is performing well
. Just don't expect it to run at 300 FPS for everything.