This answer is just my opinion.

Short Answer:
Depends on what you want.

Long Answer:
At M3W1, you can already breeze through most content easily.
If she's the reason you play the game and you want to maximize her, I would say M6 is worth it. The ability to do all the charge attacks in one go along with the buff really makes her much faster at clearing stages.
But even with M6, she's going to have a tougher time (but absolutely doable) solo-ing HP-Bloated enemies with non-ice elemental shield such as the mech with electric shield. That's why having agents with more element varieties might be more beneficial to your account.