You are correct in that the adult eye typically dilates to a maximum of between 5 and 7mm and age does tend to reduce this so that at 45 years old, it is possible that your eye might only dilate to 4mm (or even less depending on the health of your eyes and other factors.) Whatever the maximum dilation is for your eyes represents the maximum exit pupil you would need to worry about in a binocular. Of course, most people don't actually know just how wide their personal eyes dilate (and it changes over time) so we generally deal with averages and estimates and most "experts" will recommend a hunting binocular with a minimum exit pupil of 4mm with preference often being given to optics with exit pupils of at least 5mm.
Honestly, I believe that "low light performance" is an issue that is talked about and even fretted about a whole lot and in reality, is not worth anyone's time or trouble. You can measure it by focusing on the holy Exit Pupil or perhaps by the lesser known but equally powerful Twilight Factor. Once, I even developed what amounted to an amalgam of the two to try and incorporate both ideas in a single figure. But, however you try to quantify it, it just isn't as big of a deal as many would try to make you believe.
The plain truth of the matter is that just about any high quality binocular (and both the Legend and the Monarch qualify) is going to be useful both well before and significantly after legal shooting light irrespective of its absolute magnification. They also are going to perform about the same in heavy timber and other conditions where shadows are long and deep. In fact, I've even used 8X and 10X compact binoculars with great success for early morning and late evening glassing when I couldn't carry a larger binocular (which I certainly would have preferred.)
My primary full size binoculars are an 8X42 Pentax DCF WP and a 10X42 B&L Discoverer. I have also spent time with both of the binoculars you have asked about in both 8X and 10X as well as others. In my experience, neither magnification offers "low light performance" that is noticeably better than the other, let alone the idea of allowing for additional hunting opportinities any earlier or later in the day.
Pick whichever magnification you like (8X for its greater FOV, 10X for the ability to "be" that much closer to your viewing subject,) buy quality, and then enjoy. You won't really be missing anything with either choice.
------------- What if the hokey pokey really is what it's all about?
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