I'd just like to note that Kog'Maw is actually gaining attack speed with the (tentative) 6.5 changes, though with some caveats: only late in the game, and only when compared to 6.4.
I'm not entirely certain how the wiki calculated a 25.5% attack speed gain by level 18... seeing as how that brings his attack speed from 0.625 to 0.784 (yet they listed his level 18 AS as 0.88 or so, which is a 40.8% increase). Seems like someone screwed up somewhere when updating the wiki.
In any case, I did a little math based on the growth formula and here's the comparisons I've made on Kog'Maw's AS. This is assuming no runes, no masteries, no items, but maxed out Q/W (depending on patch).
Patch 6.2
0.665 (base AS), 1.5% growth
0.92 (level 18)
+0.233 (35% from Q)
=1.153
Patch 6.3
0.665 (base AS), 1.5% growth
0.92 (level 18)
+0.233 (35% from W)
=1.153
Patch 6.4
0.625 (base AS), 1.5% growth
0.88 (level 18)
+0.219 (35% from W)
=1.099 (1.061 at level 16)
Current PBE (6.5?)
0.625 (base AS), 3% growth
1.135 (level 18)
+0 (no more passive from Q/W)
=1.135 (1.059 at level 16)
So 6.5 Kog'Maw will have higher end game AS than 6.4, but it doesn't actually overtake it until level 17. Could be considered a loss, could be considered a gain... just depends when in the game you're looking I suppose. But the removal from skills does make W less overloaded and gives more consideration to leveling other skills, so I count that as a plus.
But despite all that, 6.5 is still lower than 6.3 and 6.2, and in more ways than one. The final result at level 18 is lower, but also any gains from AS items since bonus AS is always calculated off base AS, which has already been lowered as of 6.4.
Random note: Kog'Maw seems to be one of the slowest default ADCs (not counting the double AS from activating W). Many of the other ADCs have both base AS and growth greater than 0.625 and 3% respectively.
