Check the mod priority guide:
https://docs.google.com/spreadsheets/d/1ty239kyJbBs1sLhTo1JvSWuN0mIJsK1nwVdI6OLLtXo/view?rm=minimalM14 MOD plays on all of her original strengths and simply makes her even stronger. While the need for her has dwindled over time, she still stands as a decent RF.
Her primary issue before the upgrade was her low Damage stat. The upgrade helps this through a minor raw Damage stat increase and her new Skill,
Clear Fighting Spirit, that buffs her Crit Damage while her Skill 1 is active. This compounded buff brings M14’s DPS up to other powerful DPS RFs such as WA2000.
All this considered, she’s still all she was before: a strong generalist RF. That being said, with the constant influx of new top tier generalist RFs, her upgrade has become less and less attractive as time has gone on. Whether or not you consider her for upgrading will be very armoury dependent: if you have the likes of R93, General Liu, Lee-Enfield, etc., then there is really no need to upgrade M14. If you’re a newer commander or just lacking these strong dolls, M14 is worth considering.
M14 is another doll that is useful at any level of her upgrade.
MOD1 funnily carries the most value, as levelling M14 further has gotten much less valuable as time has gone on. MOD2 is still good for the skill, while hardcore fans can go up to MOD3 for the art, but it’s not worth it for her SPEQ alone.
While M14 can certainly be used against KCCO with her SPEQ-set, she is still completely replacable by better generalists that do more damage against everything.