I like options, and I like this suggestion.
I'm currently leveling up my dude Red Armor (red side, Indomitable). My goal with him is to do every contact, every story arc, every mission on the way up. To do this, he's had to slow his XP way down... only running solo, running at -1 level, and turning XP off a lot (a lot).
What I'm doing with Red is kind of extreme, but an option like what is suggested could have helped him along the way.
The suggestion here is okay. I think a better suggestion that would accomplish the same thing is a half XP mode, either clickable from the options menu or acquired from the P2W/T4V.