The stretch to the Gorges de Galamus is great for speeding.
The Gorges itself can be full with pedestrians and cyclists and if you would hit the low wall at high speed you will plunge yourself to a certain death. So slow down and get overwhelmed by the great views.
After the gorges, a small winding nice road awaits. Mind the cattle though!
Again: Speeders will not get their thrills here, but for sight-seeing it is great.
Spectacular scenery as you get towards Quillan more a road for sightseeing than racing along
An alternative is to turn right at Cubières and head on the D14 towards the Cathar Castle at Peyrepertuse, road open and well surfaced and some stunning scenery along the length of it, past Queribus (another Cathar Castle) and then ending at Tuchan.
From St Paul-De-Fenouillet take the signs for the Gorges De Galamus. This is a spectacular road along the top of an amazing gorge. There can be a lot of tourists here but if you carry on most traffic dissapears and you are in a stunning valley, with a very winding road all the way to Rennes Les Bains - a pretty village on a river. This route is quite slow but if you want to get off the beaten track its great.