For gaming? NO with regards to 480.
The 480 is a midrange gpu.
I don't game and am really just concerned with FCPX performance.
In this user case, then YES. A 480 is way faster than a 980ti, and would probably then be way faster than a 1080 if it had mac drivers.
As for the 490, no one really knows yet. I'm guessing it would be amazing in FCPX and probably good for gaming, but probably won't beat a 1080ti in gaming, perhaps it'l compete with a 1080. It depends really if AMD improves it's efficiency. Apparently Nvidia introduced more efficient game rendering techniques used in mobile phone/tablet GPU's to their desktop GPU's and that's how they got such a big performance jump from the 700 to 900 series on the same manufacturing process node.
If AMD could do something similar, then yes their 490 could then probably compete with a 1080ti.
Edit:
And if you look at DX12, it seems to favour AMD architecture. Although Nvidia is currently using bigger GPU dies with more transistors so still beat AMD in gaming performance.