On a 1920x1080p device, the navigation row is 96 pixels tall and thus takes up 5% of the display area.
For the "usable space" the M8 is approx 4.81" vs the 4.7" in the M7, so only about 2% larger diagonally, which translates to only about (10.143-9.427)/9.427 ~= 7.6% increased area.
In immersive mode or whatever they are calling it, it would be (10.662-9.427)/9.427 ~= 13.1% more area, so one could say we're losing about 42% of the total screen area increase to the on-screen buttons when they are on the screen.
For those overly concerned with this, it is possible to get ROMs or other MODs that allow users to change the DPI settings of the navigation row, limiting it's height to 48 pixels or 24, as examples or remove it entirely and replace it with PIE controls, etc. There are many options to mitigate the impact if you see this as a negative, but
no matter what those options look like, the fact remains that the S5 and G2 will be larger no matter what, so if you are trying to maximize screen size, this isn't the biggest display out there by any means.
Math for the 4.81" is in this thread:
http://forums.androidcentral.com/ht...e-up-any-larger-display-size.html#post3553025