If you truly need great low-light performance -- maybe you're a video guy? -- you might want to rethink upgrading to 50MP. You'll have smaller pixel sites, which means more in-camera software-induced signal-processing noise and more loss of light due to interstitial gaps between photosites.
For my favorite description of the issues at play, take a look at
this article on pixel sizes and resolution, and
this comparison between your 5D and the A7r/A7s, normalized for output resolution.
You should find that your 22MP 5D already outperforms the 36MP A7r in very low light, despite having a third less resolution.
That said, the heuristics for noise reduction in typical post-processing software like Aperture or Lightroom are much more sophisticated than the simple downscaling used in the comparison above.
Basically, there is more to evaluating low-light performance than looking at the ISO number, just as there is more to evaluating resolution than looking at the mega-pixels number. I probably shouldn't have generalized quite so much in my previous comment.
Disclaimer: I'm a D700 and D800E shooter.