I never loaded RL17 with a 123 grain bullet in 6.5 Creedmoor but, I have loaded it with 140 VLD’s & 142 grain Sierra HPBT’s with good success. I’m getting about 100 FPS increase over loads using H4350 with the same bullet.
For 123 grain bullets, I’ve been using Varget.
So if you are going to do some load testing with 120 grain and RL17, let us know how things go. Should be a screamer, I’m thinking that you may see some velocities well over 3000 FPS.