Are the MCarbo/Wolf/other spring kits worth it for a K9?  I'd like to lighten the final trigger pull a bit for my wife, but don't want to compromise reliability.

I'd stick with Kahr factory springs.
A lighter striker spring isn't going to offer that much reduction and Wolff specifically warns that reduced striker springs are for competition use ONLY.

Kahr's design means that recoil springs and striker springs are what they are......if you want lighter, buy a different brand of gun.
These micro guns are operating right on the edge of what's possible.  Start changing springs and they often begin failing.

If you absolutely must have lighter triggers, you can try a lighter striker spring but you'll need to do some serious test firing with the SAME ammo you'll be using in the gun for defense, and that will need at least 100 to 200 rounds to be certain.
Anything less and it becomes a matter of "How lucky do ya feel"?

Another option is to send the gun to one of the custom gunsmiths who offer high end Kahr work and have him work it over.

The best of all options is to try to get the shooter to practice more with it.  As you get used to a gun and strengthen your trigger finger, it starts feeling lighter.