Context needed. What are you going to shoot?
50mm on a 1.5x crop camera is very limiting indoors, but could be the better choice if all you shoot is talking heads.

What file did you provide to the theater? Do you know what projection setup do they use? Is that a regular movie theater calibrating for showing DCPs? Did they have any specific requirements for the material?

For a cinema projection release you either grade on a projector, or grade on a display under a known gamma setup and then convert to the required settings. Usually you'll need to produce a DCP for theater projection and in the DCP creation software you can specify the necessary gamma and color space conversion parameters. In general, you need to know the gamma, the white point and the color space primaries of the source material. For material graded on a (calibrated) computer display these are usually gamma 2.2 (or sRGB gamma, depends on calibration; 2.2 is a relatively safe assumption for uncalibrated displpays), white point 6500K, and the sRGB/rec709 primaries.

You haven't provided enough information about your mastering process but it is probably a case of different gamma settings.

If you have graded on your home monitor you have graded for a gamma of around 2.2 which most computer monitors more or less approximate out of factory. Since your display is not calibrated, it most likely deviates a bit. It is perfectly possible that it deviates slightly towards a smaller gamma value.

The projector might have been set up for a cinema projection gamma of 2.6. A film mastered at 2.2 (or less) and projected at 2.6 will suffer from crushed blacks. A side effect of increased contrast is oversaturation.

I am curious which CUDA only Adobe features do you mean?
The only one I'm aware of is the raytracer in After Effects and it is now obsolete with the Cinema4D integration.

I think you are missing the point. :)
The discussion is about shooting Canon raw video with Magic Lantern, not about shooting vanilla video with it. And there is no contest between Canon ML raw and A7s XAVC-S. ML raw destroys XAVC-S from the A7s in terms of image density. It is just incomparably better.

I wish this wasn't the case, cause I like the size of the A7s and, welll, I own it already. But I keep on renting 5dm3 for narrative, because it has a proper dense picture. I'd say 5dm3 ML raw is the best FullHD image you can find (downsampling higher resolution cameras notwithstanding).

You don't need SLI/Crossfire for 2.5K raw. The specs you've listed are more than enough to handle it in Resolve and in the new Premiere Pro CC 2015 for example (Premiere CC 2014 is much slower when processing raw).

The only possible bottleneck here is the hard drive. In theory, running 2.5K compressed raw footage from a defragmented hard drive should be ok. Going to a speed oriented raid for a working drive will improve the odds for no hickups, but it is best to try a single HDD first and only go to a raid if needed. It is surprising the continuous throughput a simple clean WD Blu can handle.

Your chosen video card is already a VERY capable OpenCL and OpenGL device so you are good in most GPU accelerated applications. Obviously, there is no CUDA support in AMD cards but there is no CUDA-only acceleration in new and recent video software as the industry is shifting towards OpenCL.