Before anybody says "lol CC her" keep in mind that I think her ult is balanced. Yes, BALANCED.
The problem isn't her ult. Rather, the problem is the 3 basic abilities that she has. They have zero cast times, no resource cost, and the cooldowns are disgustingly short. People say that she does not have kill pressure pre-6, but I beg to differ because she does. She can spam QEW with impunity due to no mana cost, and she can tower dive you and get back out because of the reset. It's really no different from fighting Akali or Fizz, and you know how toxic those champions are. At least Akali and Fizz are gated by energy and mana.
And as for "lol CC her", what that basically means is that if you're fighting her, and the skill you have with the CC is an important lockdown/damage/escape skill, then that means you're stuck using your other 2/3 basic skills until she uses her ult. And by that time, she may kill you before she even has the luxury to ult you. This means that fighting Kat is like as if she disabled one of your abilities, in this case, the one with the crucial CC, which you'll have to use only when she ults.
On a side note, I think champions that do not use mana or energy are toxic in general? Why? Because they can spam skills left and right to their heart's content and never ever worry about running out of mana or energy. And their build paths are far superior too, since they don't ever have to worry about itemizing mana. Most mid-laners have to rush a chalice and spend a big chunk of gold buying mana items causing you to fall behind in damage. Kat doesn't.
Her q has a cast time and that is the reason you cannot use it in your dfg combo, it is what holds her back from being a legit single target assassin. In my opinion it is the reason Katarina cannot see serious LCS play. Her q and her e have really long cooldowns at low levels, only w (sinister steel) has a short cooldown. You are correct that these spells have shorter cooldowns at higher ranks though.
Every champion has kill pressure before 6 if their enemy allows them to have it, there are very few champions kat can trade with pre 6. At level 1 her w spell does "40" damage. She has 56 ad, so she would be better off right clicking you instead of spamming sinister steel on you. Her base damages are really really low due to the fact that she has "resets" and no resource.
You dont just cc her to stop her ultimate, you do it to kill her. Katarina cannot build defensive items early due to needing to snowball to be relevant. When I play Katarina I hardly ever duel. WHY ? Because that is not what Kat is built for and it is not what she is good at. There are very few champions that katarina wins duels with.
I believe that a fed Katarina is easier to shutdown than ANY OTHER FED CHAMPION in the game due to her kit and the way it works.
Please dont call Katarina toxic, she has no cc at all. She brings damage to the table and nothing else, she also does this at the cost of having good wave clear / siege potential. Katarina is a greedy pick because she requires her team to engage and for the most part be winning the team fight before she becomes valuable. Katarina is poor at actually taking objectives (dragon,baron,turret) and takes the slot of your normally safe ap mid laner and jumps in the middle of a team fight with reckless abandon, hoping no praying for a assist or kill or she will die and her team will lose.