Hey John - I'm using the nvidia-340 drivers. In the Nvidia X Server Settings (under PRIME Profiles) I have it set to - "Select the GPU you would like to use - Nvidia (Performance Mode)" In the left hand panel in Nvidia settings its listing is GPU 0 - (GeForce GTX 880M) so I'm guessing it is using the correct GPU as the Intel isn't being used as far as I can tell. I've recently found its not just web based games that have artefacts, but movies have a jerkiness to them especially when panning left to right or vis versa. I get none of these issues with Windows, so I'm guessing its not hardware related. Real shame too, absolutely love Linux Mint.
That's pretty much how things are. Windows 8 for gaming and Linux Mint for everything else. Just a shame I'll have to use Windows for movies too.
Just my 2 cents, but Firefox is a terrible choice to use to benchmark/test anything. It has lately become a very bloated resource hog, and the past few releases have been quite buggy. I would use a game and/or a office document to get a better view of the highs and lows.