Bootcamp is fine if you want ultimate FPS, but it's a pain in the neck rebooting regularly. It depends on what you call "casual gaming". I play a fair number of games, although they're usually the less up-to-the-minute ones and not so graphically intensive. Half-life 2, Portal, Civ 5, AOE HD, Doom 3, Minecraft, and that sort of thing play perfectly smoothly in 5k on OSX (the AOE HD is in a parallels window running Windows 10, and so can't run higher than 4k but can do that in a window).
Once you've run games in 5k, though, you will be spoilt for lower resolutions...
Once you've run games in 5k, though, you will be spoilt for lower resolutions...