On clear river with a high fishing pressure, I only use 6 to 7 meters long Maxima tapered leader, not the easiest to cast but it offers a perfect presentation for a small nymph.
Caught those 2 beauties last saturday on sight with such a leader
I caught several asps last summer, the attack is really impressive, the fight not that hard. The most difficult thing is to cast/ stripp as quick as possible an entire day, really tiring at the end of the day.
Should get in contact with fishing guides, like :
Accueil Christophe Pironnie, Guide de pêche dans le Verdon
Some incredible big trout coming from the lake of "Ste Croix" (up to 10 kg), but fly fishing isn't the most appropriate method for them.