Good question. In terms of low light performance, exit pupil is the dominant factor. The difference in magnification won't make up for the light loss. That's apparent from the luminance contrast curves for the human eye.
As long as you're comparing optics in a similar price range, the difference in coating transmission is not more than 10%. However, that won't apply comparing, say a $200 8x40 roof prism bino with a $1,000 bino. In that case, cheaper coatings will have lower transmission, probably by as much as 30%. Now the difference in low light performance will be harder to notice.
I've heard the older Steiner binos had low transmission coatings. I don't believe the newer Predator binos are that bad. I'll look for transmission data when I get back to the office.
There are other reasons for buying the higher end optics. Transmission is higher for high end optics, but not by as much as you would expect, because those products usually have more lenses. The light loss per lens is lower, but the light has to pass through more lenses. Resolution across the field of view, usable field of view, and color accuracy generally improve with price. Ergonomics improve and fatigue is much less for higher end optics.
Fatigue is hard to quantify. It has to do with collimation, usable field of view, resolution and other factors. I know from personal experience that classing for two hours with a cheap binocular is almost impossible for me, while doing it with a Swarovski SLC is almost effortless. The higher end bino allows me to focus attention on the scene, while the lower end bino forces me to focus attention on the process of classing and managing eye, neck and shoulder fatigue.
