So, there's quite a few things I've noticed when level grinding.
1. Possessed Galiene doesn't have a speed limit. She is already at max level here. 
Base Galiene is also at level 90 with 129 speed. I haven't checked if base Galiene could continue to grow her speed, and I haven't bothered to see if other possessed characters (Cephrina-Evie, Cephrina-Selena, etc.) can continue to grow or not.
2. If you max level possessed Faylen, tell Cindy to release her body, then possess Faylen as Celica, her level decreases when she gets re-possessed by Cindy. Most likely averaging the levels between Celica-Faylen and Cindy-Faylen.
Before telling Cindy to release:
After possessing as Celica, then re-possessing as Cindy:
You'll notice that Faylen's speed is quite high at this level, and I imagine it will most likely surpass her previous speed of 161.