I'm wondering what the point is of 8x10, other than the ability to contact print and the ability to view a larger groundglass.
I think you've got it.
Compared to 5x4, 10x8 has the following pros and cons in my book.
Pros:
1) More film area, thus less enlargement for the same size print. A 125 x 100 cm print is roughly 10x for 5x4, and 5x for 10x8, so the print will be less grainy. It may not be any sharper however because of the smaller aperture required for 10x8 to get the same DOF.
The larger film area can make for better tonality of smaller elements like a distant tree trunk. In 5x4 it might be rendered as just a uniform line, while in 10x8 it might be rendered as a thicker line with shading to show the roundness.
2) Nice size for contact prints, so doesn't require enlarging.
3) GG size may be a "pro" or a "con" depending on the user.
Cons:
1) Heavier, a big concern if you backpack.
2) More expensive (film, processing, film holders, etc.).
3) For a comparable lens/aperture, considerably less DOF.
4) For a comparable lens/DOF, a considerably smaller aperture and thus diffraction limited sharpness.
5) More problems with film flatness which again can effect image sharpness.
6) Requires more space in the darkroom for enlarging. 10x8 enlargers are... huge. And tall.
I've analyzed the hell out of this for years trying to justify moving up from 5x4. And the analysis always tells me to stick with 5x4. So I have. For me, for what I do, 5x4 is the "sweet spot" of photography.