EDIT: I wouldn't start to level up boosters if I was you because there are two new legecies coming up.
Same as I did when I reached the level cap back in December..play the game.
Don't you go back to eating kittens?
Srsly... I screw around maxing boosters I usually don't care about. I'm working on "Come Here" this week, then I'll go into TDM/FFA and work on Ammo Award. By which I mean play the game and see if I get the requisite perks.
I don't bother with them, but you could do the missions.
The money for them is pitiful. I have 5, 000, 000 unspent, what good is $9, 000, especially when, even if you finish them all, you get no additional reward...