Yes these is asphalt on the R307! BUT all the way on top these is a stretch of a few kilometers where to road is totally broken! I didn't have a 4x4 but a normal car and I did manage to get accross but I can imagine that if the grounds are wet or snowy, it will be dangerous and hard. You definitely don't want your car to break down there because it is very very far from any possible help and not much traffic comes by either. I wouldn't go there in winters unless you have a 4x4. The R307 between Demnate and the N10 is absolutely beautiful!
I also took the Tizi 'n Test which is also a beautiful road and much better maintained than the R307. If I have to choose, I would take the R307 (only in summer) because it is less crowded and more 'dangerous'.
When are you going??