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His ult can, in fact, be improved to 0.6 sec CD with max CDR at level 16. So, as you said, CDR and Mpen should be top priorities.
So while I'm withdrawing my support from the AA staff stacking that I originally suggested, I think that people underestimate AP on Kog. With that same 0.6 sec speed, 1 point of AP will deal (0.3 AP / 0.6 sec) = 0.5 AP per second. Compare that to a 1.0 AP skill on a 3.6 sec cooldown (Nunu's Iceblast at max CDR), and you only get (1.0 AP / 3.6 sec) = 0.2777 per second.
Now, you obviously won't hit every ult, but it does AoE damage, has a CRAZY range, and you will hit a good percentage of them in a teamfight with cc flying around. And don't forget that you have three other skills that also have decent AP scaling. So I think think that an Archangel's staff is a good investment. I also think that building pretty squishy (maybe getting a Glacial Shroud/Frozen Heat, because it's so **** good) is the only way to play him, since he's actually SAFER than as/onhit or AD kog.
Oh, and one other thing to be said about stacking AP: I can't think of another champ that can explode a squishy, then do it again seven second later.