IMO the grip's going to be the deal-breaker there. You could keep the gripped body and separate lens in the F5XC; in order to accommodate the lens in the F5XB, the body would have to go in one end with the attached lens reaching across the length of the bag. The grip would make the body too wide for the bag. You'd see the same issue with the F5XC and body with attached grip and lens; although the assemblage would fit vertically, the grip would bulge the bag too much.
You might look at the somewhat larger (mainly deeper) F3x or F6 bags.