Malz's ult is a clear counter to Kat's ult, so the outcome there is obvious. Laning phase, a smart kat is not going to try to actually kill a malz when he has any considerable amount of health, especially not with her ult. Her ult will NEVER do its full damage, unless the Malz is mentally deficient or his R key has been removed.
In the end, it will go to whoever is able to poke best, and a smart Kat wins that one. Everyone is comparing Malz's minion jumping ability to Kat's but the problem is that Kat can see Malz's coming from miles away while Kat's hits all its targets within a second. She will poke more with her knife and Malz won't be able to stop her from shunpo + spin to break the knife seal. His silence is fairly easy to dodge and his pool is a no-brainer. A good Kat can harass and get some minion kills at the same time without taking much damage from Malz. Unfortunately most Kat players are not good, they're greedy.
To finish Malz off after poking, I bring exhaust and flash when I play Kat rather than ignite for these very reasons: once Malz is low from poking (and IF I'm still high hp from dodging his pokes), combo, shunpo in, exhaust, and ult him. He has a choice: die from your ult or do 70% damage with his ult (or flash out, which everyone can do...). After poking correctly for a while, you should be able to survive the 70% ult and hopefully take out his remaining health with knife and spin which will be off cooldown once his ult is over.
I guess my conclusion would be this: a smart Malz vs a smart Kat will probly be a standoff, but a successful strategy from Kat could give her a kill or two. A failed strategy will give them to Malz. Malz is the bully and he NEEDS Kat to initiate to shut down her ult, so it's all on Kat's shoulders whether she outsmarts him.