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Increased reflectivity in the main mirror has a 100% improvement translation into the viewfinder exit pupil. I.e, if you increase reflectivity by 20%, the VF picture also gets exactly 20% brighter. Relative light loss (in percent) is the same for all reflectivities.
The matte screen does not have an effective aperture in itself, it's just a lossy translation (diffusion) medium connecting the VF aperture and the lens aperture. The resulting projected image brightness is a complex angle interaction, an integration of space angle loss sums. Higher dispersion matte screens can project light from the rim of larger lens apertures into the VF aperture as more matte means "larger diffusion spread angle". But it also increases losses, as light from the center of the aperture is spread outside the VF aperture. The average light-loss for the entire (large) lens aperture throughput is fairly constant, but the aperture-edge light-loss goes DOWN. You get better DoF-conservation, but a slightly less bright image in some cases.
A low-matte screen makes sure that more of the lens-aperture central rays (chief rays) gets through unhindered to the VF (isn't scattered outside the VF entry pupil), but it also loses a lot of the aperture-rim light (peripheral rays) since it doesn't allow for enough diffusion - random angle bending of the rays. Longer DoF in the VF, less accurate focusing, but a bright image even with smaller apertures.
Unless you introduce some shaping optics into the equation, the maximum brightness for any combination of lens aperture > screen > VF aperture is fairly constant (!) - you just choose the lens F-no where the system is at "maximum light transfer" by choosing the amount of (and type of) matte. Generally you can say that the system has an efficiency pivot, where light throughput starts to decline. High matte screens start falling off in brightness at lower F-no's than low matte screens - but they can also start from a slightly higher point if they're correctly designed for the system.
The mirror is totally separate from all this, it's a linear loss medium.