I don't have a 5D IV, but I have both the 5D III and the original 7D, so I can speak to the difference.
The 5DIII is a superb camera. It certainly beats the 7D in low-light performance, it's controls and customizability are great, and the AF system is also excellent. It's what I use 100% of the time when I am doing candids of kids and events. And you're right: the lenses you have will pair very nicely with a FF.
What you lose, relative to the 7D, is mostly reach. Because of the difference in reach, you are going to be carrying heavier and more expensive gear for any given framing.You also lose pixel density, but for most people, that doesn't matter. I do macro, so for me it does. I keep my 7D for macro and for the rare cases where I want more reach than I can get with a FF, a 70-200, and a 1.4x extender.
From what I have read, the IV is a step up in several respects, but not enough that I personally am tempted to spend the money to trade. For others, the decision may be different.
get Mk iii

Of course - its nothing special and new...but just wanted to share...
