The game is horribly hungry, memory-wise, and doesn't use the entire VRAM either (if I recall correctly, 85%) unless you tweak the config file manually. As an extra, ultra details cause some serious memory leaking on MP and SP, albeit they have a reduced effect on the latter, so drop to high if you are trying to run ultra. A 270x should be fine for up to high details if you take supersampling out of the equation and use FXAA.
If the stutters go away after moving around the map a bit and if they're stronger only during the first match, then it's pretty much normal. I have them too on a 4gb 290x tweaked for 95% VRAM usage (as 100% causes out of memory errors...if you're interested in the rest of the system for comparison, specs are in my sig).
Let the system warm up to the game with a couple matches and the only lag that remains is the one caused by the lack of dedicated servers
In the meantime, I'd give a try to the latest 15.2 beta drivers. They might not fix anything, they might worsen the issue or they might make a miracle. Just keep in mind they're BETA DRIVERS, so you can't really blame AMD if something goes wrong or works bad...
Last, but not least, to answer the "options greyed out" issue, you can only change them in menus. They get disabled when you're in a match, showing as greyed out.