I went from 7D to 5D3, but never really bothered about the crop as I also changed my lenses to suit.
The few occasions when I do need to crop, I have had no issues with image quality cropping down to even 100%. However if you already crop on the 7D then gong full frame without changing lenses to match may not be the best option for you.
Why are you thinking of going 5D3?