In 2015, the French magazine Nat'Images published a test of 60 Binos.
They used the lab and optical machines their twin magazine Chasseur d'images have had made (and improved), starting in the 80's. Their optical tests are a reference, and Nikon and Canon are trusting them, as the magazine proved them many times the flaws of some of their lenses.
So, in their opinion, for some models, there's a difference optically between 8x and 10x of the same diameter models.
They claim increase magnification decrease quality (for some models).
For example, the Leica Trinovid and Ultravid 8x42 are better optically speaking than their 10x42 counterpart.
The same goes for Zeiss Conquest 8x32 better than the 10x32 Conquest.
Same goes too for the Terra's and Swarovski CL 8x30 Companion (not the new ones).
Exception for the Pentax S Serie WP.
The 10x42 are better than the 8x42.
Note that the Bushnell Excursion 8x42 HD has been declared the surprise of the test. Very good optical quality, Swaro EL 8.5x42 and 10x42 at the very top of the Test, Zeiss Victory SF 10x42 behind.
Writing all this, and knowing Quality Control, I think it's difficult to generalize about something like that.
I mean, what if the tester receive a cherry 8x and a 10x lemon...