Re: NightForce NXS 12-42x56 Illuminated NP-R2 Reticle
I find that 4'' sunshades darken the sight picture too much and therefore, use only 2 1/2" shades. I keep a threaded on both ends 4 inch shade in the ruck for extreme circumstances such as fresh snow in bright sunlight, but the vast majority of the time the 2 1/2" shade works best. Even during night fire I only have to remove it on moonless nights to see the targets, with the magnification set on the lower powers - of course.
I am aware of no add on filters to enhance image brilliance outside of nighttime observation devices, but know that Leupold used something they call "Multicoat 4" that is supposed to serve the same purpose. It's nothing you have to buy, all Leupies ship with it applied at the factory.
In short, I don't see a magical or mechanical solution to solving your problem of darker image at ultra high magnification. It's all a matter of engineering, and the only way the engineers can brighten the image is to increase the size of the objective or decrease the maximum magnification. Again, a give and take scenario, and no gain comes without a cost.