You could probably edit the file I mentioned above and experiment with numbers >1, e.g. 1.1, 1.01, 1.05, 1.25 and see what you can adjust and how far you can take them. Would be really interesting to see your results.
Presumably they do either detect your HMD, or the mask is designed to work for most HMDs based on the display resolution or aspect ratio, but I've seen a lot of people posting talking about different results with the same headset, so I wonder if your individual eyes play a role, and/or the shape/size of your head and how close to the screen your eyes end up. I know some people modify G2s to get the screen to sit closer to their face to get a bigger FOV, but I feel like mine already sits as close as I would ever want it.