Check the mod priority guide:
https://docs.google.com/spreadsheets/d/1ty239kyJbBs1sLhTo1JvSWuN0mIJsK1nwVdI6OLLtXo/view?rm=minimalK5 being a desirable Neural Upgrade should not be a surprise to anyone familiar with her. She is already a great HG, and only gets even better with her upgrade.
Her new Skill 2 is Astrological Divination, which grants buffs depending on the types of enemies present on Skill 1’s activation. If there are any Elite enemies, she grants all units a 12% Rate of Fire buff and 50% Movement boost for Skill 1’s duration. But more notably, if there are High Threat units present, she gives everyone a 100% Force Shield for one second. On top of this, she gets the typical array of upgrade bonuses like stronger tiles, and her Skill 1 now grants additional ammo to MGs and SGs.
The “High Threat” tag includes every single boss. With this, K5 can make the entire team shrug off any massive boss attacks she wishes, provided the skill activation is timed correctly. A very straightforward but extremely potent tool that few dolls can provide for sure, which does also mean K5 will want to be used on manual skill for such fights, so you can time this as you please. The Rate of Fire bonuses that Skill 2 provides are also a pretty nice buff to almost anyone, and will happen fairly frequently as elite enemies are pretty common.
Overall, this is simply a case of an already strong doll getting even better in a big way, so K5 is very much worth your time. Raising her to MOD2 is the ideal stopping point. MOD3 offers her very little on its own, but with the introduction of her SPEQ-Set, it can be considered. Her SPEQ-Set adds 10% to all the bonuses on her Skill 1 and 2 when there is a boss on the field, making her just an extra bit better.