Yeah, I think her Q spell needs a buff, more than anything.
Right now I'm finding the best way to play her is to concentrate on her burst damage by going the AP route, and that leaves her Q woefully weak.
She doesn't really benefit much from DPS items as she's pretty weak in a straight out 1v1 fight - she's better played as a finisher/chaser/sniper in team battles.

Plus the W + Q combo is nowhere near as useful as her W + E combo, even if you go the DPS route. Healing Reduction is practically useless.

Edit: Also I agree with EternalOne about her not getting a stun or slow. She can already get those via Exhaust/Rylai's Scepter/Frozen Mallet, can already chase very well with Shunpo, and her not having a reliable snare early on makes her more reliant on teamwork - which is always a good thing.

Her Q spell is that weak because it's FREE and has an 8 second cooldown. It's purpose is last-hitting and harassing on the lane. And of course anti healing when in team battles. Nothing more.

Sure, except that doesn't explain why her E spell is so strong. 90% of the time I'd rather combo KI with Shunpo rather than Bouncing Blade. Healing Reduction is not something you will need every game.
I wouldn't mind a longer cd on Q if it would slightly scale with AP + have a better debuff like -MR/Armor.

1) 600+ AP means you have very little in the way of tanking items, and Kat is a very fragile hero without them.
2) 600+ AP means that the game has been going on for very long, a lot of heroes become insanely strong if they stack that many items.
3) Her ultimate can always be disrupted by a stun or escaped from by getting out of its tiny range. Even if you're slowed you can always Flash out of there. Then again, she can always use her ultimate again very quickly so this is a minor point.

I don't see how you can escape her ultimate with flash considering the insanely different cooldowns between flash and her ulti. Especially in team battles when after a guy dies it's almost done again. If you do not have a stun which many champions lack you're supposed to do what? Stick with a stunner just because this champ exists and her AP stacking works too well? Add a rilay's and time to chase with shunpo 1:1 ap ratio.

I think she's balanced. I would maybe change her W ability actives, but otherwise she's good as she is.

with rylais does her ultimate trigger the 40% or 15% slow?? really seems like the 40% and it is really insane..