First, a big thanks to Gizmo for providing lots of screenshots. Without them, I wouldn't have been able to figure out how the widescreen works.
About the game, this may be the only game where 4:3 has the wrong aspect ratio. I've looked at three widescreen shots, a 1280x720, 1280x768, and 1280x800, and the geometry visible in all three had the same proportions. However, a fourth screenshot at 1024x768 had different proportions from the widescreen shots. This either meant widescreen mode is stretched vertically, or 4:3 mode is stretched horizontally (I assumed the former at first, but later pondered the possibility of the latter).
So I asked for some more screenshots, a 1024x768, a 1280x1024, and a 1280x800. The 1280x1024 had the same proportions as the 1280x800, but the 1024x768 was a horizontally stretched version of the 1280x1024. This confirmed it: 5:4 is the correct aspect ratio for non-widescreen, and 4:3 resolutions just use a stretched version of that.
So, here is the comparison:
1280x1024:
1280x800:
And just for the hell of it, 1024x768, which uses the wrong aspect ratio (it's a stretched 1280x1024):
Comparing the first two screenshots, you can see that the widescreen shot loses some V-FOV and gains some H-FOV. Here's a crossover image with 5:4 ontop of 16:10. Details above and below the red lines represent vertical FOV lost, details left and right of the yellow lines represent horizontal FOV gained.
A lot more V-FOV is lost than H-FOV gained. Therefore, this game is graded as if it is vert -, and gets a C+.
Also for the hell of it, a 1280x720 by 1280x800 comparison:
1280x720
1280x800
The 1280x720 (16:9) picture is perfectly vert -.