I don't think you'll get that much more out of the 50L than the 1.4. On the other hand, you're missing out on good glass at the wide end.
While the 1DX is a superlative camera, there actually are situations where the 5DIII is the better choice even if money is no object. The 5DIII has an ultra-low-noise shutter mode that, by itself, makes it a far superior choice for wedding photography as well as anything else where silence is demanded, expected, or appreciated; in those situations, there isn't anything that the 1DX actually does better than the 5DIII, making the 5DIII the clear winner.
Similarly, the 5DIII is smaller and lighter. Take off the grip, stick a Shorty McForty on it, turn on live view, and it basically looks like an advanced P&S and nothing like the monster it really is. It'll even fit in a cargo pants pocket. If you're looking for a party camera on steroids, that's it.
The 5DIII also has a resolution advantage, which is important for focal length-limited work. The 5DIII might be a better camera for birders; it's certainly not a worse camera.
Where the 1DX reigns supreme is for action photography. If you're shooting for Sports Illustrated (or will be pretending you are), you'd be insane not to get it. It also does a bit better in low light shooting (but not hugely).
But, for everything else? The 5DIII is an equal or better choice -- and, make no mistrake: it's also awesome at sports and low light, just not quite as awesome. A decade ago, the 5DIII would have eaten the lunch of every other camera on the sporting green. Sure, the cameras back then had somewhat faster frame rates, but the 5DIII's autofocus means more in-focus shots per second for an overall win.
Hopefully, that's enough to help you make some decisions....