I traded in my pair of 7D's for a pair of 5D Mark III's (I work professionally with newspapers and magazines). For my performance photography, I wouldn't say the 5D3 has added much in terms of high ISO performance for the price. Sure the files are cleaner (maybe 1-1.5 stops better), but I don't think they are $2,500 better.
My mindset is that my clients can't tell the difference between photos shot with the 7D and 5D3 at ISO 6400. They also can't tell the difference between a photo shot with the 50 f/1.4 and 50 f/1.2L. Therefore, they are going to buy your prints regardless of your equipment. What's in the frame is what matters, not the high ISO noise.
Your best bet is to buy a faster prime lens rather than spending money on a more expensive body, IF your only hold up is high ISO noise. You can snag a 135 f/2L for $800 used (in like new condition) and take your settings from 1/320 sec, f/2.8, ISO 6400 to 1/320 sec, f/2, ISO 3200. Need something longer, for not much more money than a new 5D3, you can buy a used 200mm f/2L IS.
Here is a youth ballet performance I shot in 2011 with the 7D and 70-200 f/2.8L IS II combo – http://soskiphoto.photoshelter.com …4ruXPu9A/C0000cgq5MZj4G_0
And here is another [Similar] performance I shot last month with the 5D3 and 70-200 f/2.8L IS II combo. You be the judge if the noise difference is noticeable. – http://soskiphoto.photoshelter.com …EOQZ4yGw/C0000jOj4H9Aa0yY
With that said, I hope it helps steer you in the right direction. I absolutely love my 5D3's, but I also loved my 7D's for their price point. To make it clear, I upgraded to the 5D3's for my general daily news assignments, the full frame look, the autofocus system, and because my employer was paying for it. I did not upgrade for the slightly better high ISO performance.