@Meddler: Targeting Kog'Maw's base attack speed is a net nerf. Please don't.
Now I'm a big fan of the whole constructive criticism protip that most writers/artists abide by: "If someone tells you something seems wrong, they're most likely right. But if someone tells you how to fix it, they're most likely wrong."
I won't get into the specifics of why that statement holds true, but this is why I'm not going to propose any balance suggestions. I'm not a game designer, I'm not a balance expert, I'm not even all that great a LoL player. But what I am is a Kog'Maw player. So I'm identifying what seems wrong, then I'm trusting Riot's balance team to fix it.
> {quoted} > > Regarding 6.3 it does look like we've overbuffed Kog in general. Looking into options at the moment, with base AS the most likely target.
So I don't think this is right. Apparently moving the attack speed buff from Q to W made Kog'Maw too strong early, no real change late (I mean, I don't mind but there are so many complaints, it seems). Since you've been saying the idea for Kog'Maw was to be a true late game hypercarry, patch 6.3 really didn't do anything toward his end goal; it tried to make him good at something else entirely (his early/mid).
By level 18, there's effectively no difference between pre6.3 and post6.3 Kog'Maw, so keeping this change and then lowering his base attack speed is simply a net nerf to Kog'Maw.
As such, I don't believe it's wise to keep what didn't work, and then change more things on top of it in an attempt to fix it. That only tends to exacerbate what went wrong in the first place. Normally, when you take a step forward and it didn't seem to work, you should take a step back and review alternatives.
Again, I don't have any idea what you should do to fix Kog'Maw, just that this current path seems wrong.