PSA Do not play Kog bot
As of patch 6.21 there is absolutely no reason to play Kog as a Marksman, he simply does not have the kit to support that identity. With the mana cost of his ult reverted back to 40, base damage increase, and the possibility of a triple damage execute removed Kog has become a better mid lane poke mage than anything else.
AD Kogs itemization is absolute garbage, his core items are expensive
and don't provide him the same power as other Marksman who itemize similarly. His 500 base range leaves him open to harassment or creep blocked while trying to last hit effectively. He has no effective wave clear or shoving power, no utility, and no strong synergies with ally supports. Additionally his late game is average at best sporting a lower damage late game than Caitlyn, Jinx, Twitch, or Tristana and worse early games than literally every other marksman on the block.
On the other hand AP Kogs itemization bolsters his health quite a bit and allows him to play from a much safer distance than with AD Kog. You still suffer from the absolute shit early game, but once you hit 6 you can begin wave clearing with ease
will help keep your mana high and later turn into a powerful defensive item
. From this point on you can play keep away with your opponents taking pot shots at them while they are at a distance and using Void ooze to deter retaliation. AP Kog can wave clear and shove. His late game is on the level of varus or Jayce in terms of poking power.
I love playing Kog as the tank shredding carry Riot thinks he is, but right now and moving into preseason if you play Kog as a Marksman you are handicapping your team.
Edit: This thread is a lesson in how one person can be so wrong