I am considering getting the 1dx for manual exp comp, better tracking, cleaner iso images 3200-6400, more customizable features with the second mn function button.
Is this worth $3k+ to you? I think the biggest gain with this upgrade would be 12fps, but I get the impression these are more wants and nice-to-haves.
Get 16-35 2.8 ( for landscapes, large groups)
More backdrops, lighting, etc
300mm f4 is
1.4 ext iii
Canon 8-15 f4
Based on what you like to shoot, I think you can do a lot more with the above. The 1DX and a 50 aren't going to let you do a whole lot more than what you're likely capable of shooting now.