I've driven from Calais to Nice many times, quite a few of them around the 14th/15th August and to be honest it's no different from any other week in August. Busy, but not dramatically so. It's not like the roads empty miraculously on the 16th <wishful>
Try to avoid 'Black Saturday' which is the busiest travelling day of the year as holidays start. this web page. I am hoping the PDF will link to the busy days for travelling, we use one every year to decide when to stay put and when to travel! Saturdays in August on the busiest routes can be horrific I'm still blanking the journey we did two years ago, I've never been so scared in a car. Plus there are lots if jams. Most of the time the toll roads are fantastic though, as was said, much better that Britain.
Is it to be avoided at all costs? It's our first trip driving through France and we were going to take Eurotunnel on 15th August and get a few hours drive in before a stopover, and the following day carry on to Sarlat area (SW).