On mid I'm really liking spellbook (with aproach velocity, if I'll build rilay), and sometimes RoA. Kog rly don't need manaflow band if he can get biscuits(it gives you 160 mana), and he's R don't scale with CDR. As well, the best atribute on him is magic pen 'cause the only great scale with AP that he have is on E. So, with this on mind, rabadon isn't that gold effective.
Btw, I prefer starting with safira cristal + refil. You just miss 240(2 waves almost) gold w/ doran's and 100 with red pots. As you're a scaling control mage, I prefer just loss 90(4 minions) on refil and go B with 475, so I can stack my tear faster on the game. After this I rush 2 ruby cristal, it gives you so much life, and its rly hard to kill you. Oblivium orb (morello's small item, idk the name but think it's right) is the most gold effective item on ap kog, so I rush it and then finish my archangel. I rly don't like banshee, as I sad, brute AP isn't so effective on kog, and if someone like akali or katarina dashs on you, you're dead. If you think you got some kill pressure, you can start with corrupting and warp time tonic.
Just to be clear, this is my playstyle. I'm not saying that you're wrong or something like that, I'm saying to you what I prefer and you might like as well :D