After Playing 3 Days of Kog Maw the W's mana cost is still bothering me due to losing kills in the beginning. It seems to me that the E(Void Ooze) having such high mana is what screwing me when we fight and/or when a jungler goes in for a gank when I'm already half way into my mana making my W being unable to activate.
So i'm positive some mana changes is needed for him. I believe in having no mana cost on W, because the q,w,e, combined after trading is most likely gonna burn the chances of my ulti going for another hit. And in late game I only use the ulti for not poke but to finish off running opponents because spamming that quickly burns mana. The meta is too fast paced for the slowest adc in my opinion and the farming is still horrid being far back in lane. Before the adc update Kog Maw wasn't really op when he had a no mana cost W.