RX 580 ARMOR MK2 8G OC vs MSI 1070ti Gaming X

Ah, I meant achieve in terms of more fps, or, say, max details at a lower FPS.

If you want to max your frames per second, go with the 1070Ti.

If you want max details at 60fps, stick with your RX480 and keep your money. As WildCard mentioned, the RX 580 is only a marginal improvement over the RX 480, so, there'd be only a small number of fps increase in doing that. Wouldn't be noticeable, and I agree with WC in that it's not at all worth the expenditure to go from 480 to 580.
 
Don't worry about bottlenecking. People generally obsess over that. The better video card will increase performance for most games, if you're looking to increase your fps at your current resolution.

Nothing can possibly perform less than it does currently with the upgrade.