I had never thought about the idea that there might be differences in optical quality between a manufacturer's 8x42 binocular and 10x42 binocular in the same model. I take it this was in fact your experience with the 10x42s?
In this specific case, yes, there was. As I mentioned, I bought one of the black 10x42 models and found it to be above average for that price range in terms of optical performance. When I bought the camo version 10x42 this past November I was very disappointed in the optical performance as the spherical abberation was horrible. Both pairs of the 8x42s I have had the chance to look through did not exhibit the same levels of spherical abberation.
This brings up two key points. One, quality control at this price point is not as good as the more expensive models so it is almost imperative that you actually try the exact pair of binoculars that you are going to walk out the door with. Not just the same model but the exact unit that you are buying. Two, it is much simpler for a manufacturer to create a decent quality 7x or 8x binocular. When going with a higher magnification binocular any deficiencies in the optical quality are "magnified" (pun intended) moreso than in an 8x or 7x model.
Your references to astronomical use of binoculars is spot on and a perfect illustration as to why a higher power binocular may not always give you more detail under any given condition.
I am glad to see you bought the Monarchs. I do think they are a good binocular in that price range. The only pair that I personally think are roughly equivalent to them in terms of quality, size and price are the Bushnell Legends.