AD kog is sub-optimal. I will explain why:
As AD kog, you will be getting items that prioritize attack damage, crit, and maybe lifesteal, rather than attackspeed and ap. Becaues of this, AD Kog'Maws will generally be dealing a mix of both ap and ad damage. The problem though, is that AD abilities like crit and lifesteal don't stack with the AP damage of your Arcane Barrage. The other problem with AD kog is that he will generally go either armor pen or ap pen, but either way, the effect is crippled if you go AD. If you get last whisper, your ap damage will be quite low. If you get void staff in an AD build, your AD won't do all that much dmg.
So what should you build?
Items like malady, bloodrazor, and void staff are great for Kog. The attackspeed increase scales with his W because you will land more attacks with it. The AP items will increase the % damage of W, making you shred tanks even more. Void staff + 15% pen from talent tree will make you deal roughly 6-7% pure damage, per hit. That's Vayne's silver bolts x2.5.
Avoid items like IE, Bloodthirster, and phantom dancer, because there are better alternatives. Kog does NOT need crit or lifesteal. EVER!