Definitely not. My 580 does not preform as fast as a 670 does under no circumstance. I don't have any issues of that kind, so I have to say that there could be something up with the game itself.
What kind of a drop are we talking about? Like from 120-40 or 120-110? Is it substantial and noticeable Or do you only notice it while you have FPS being displayed? I'm going to hop into a match and do some investigating for you.
Also, is vsync on or off? If it's off, try turning it ON. If that doesn't resolve the issue, I would suggest going to your NVIDIA control panel and making sure you do two things:
1) Update GPU drivers.
2) Try using adaptive vsync. This will essentially disable the limiting of FPS until it hits your maximum screen refresh rate, then it will turn it on right away so you don't notice screen tearing or seemingly lower FPS (even though it's not really a drop in FPS, it looks like it is)
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