AP Kog makes more sense to me since every single one of his abilities deal magic damage and scale with AP, and are all always effective in all situations (except Q, arguably, because of it's short range). Hell, AP Kog used to be more powerful than AD Kog before his ratios and cooldowns took a hit, and he was given a passive AS buff. Also, he has much longer range and the only limitation on his ultimate is mana, rather than a stock of 7.
AP Ezreal makes sense, because his Q applies on-hit effects (lichbane/fist), and his other three abilities scale with AP (very well, with a .75, .8, and .9).
I never said AP Corki was an assassin... the Kassadin comment was just a sarcastic shot comparing Corki to a powerful AP Carry that people often complain about as being overpowered (at least outside of SR).
Corki has two mediocre AP ratios at .5 and .3/.45. Yes, the ultimate is cheap, long range, and spammable, but it's limited to 7 in a burst, which admittedly will do a nice piece of damage assuming you land all of them (and they hit a viable target before exploding prematurely on a minion or tank). His last AP scaling ability is dependent on the enemy chasing you through giant circles of flames... and, needless to say, enemies typically avoid the big red circles, in my experience.
Again, if you're having fun doing it, more power to you. I just don't want there to be any false expectations that Corki is a powerful AP Carry the likes of those built to do so. On TT, I agree, AD Carries are not in the best place right now, which is why the meta is 2 bruiser, 1 AP. If you want to maximize your effectiveness on the team, my suggestion would be to choose an AP Carry that can do the best job at that, with good AP ratios, an escape, and utility. Corki has the escape, but lacks AP ratios and utility. He's fun to play, and that's a fine reason to play him - just let your team know .