How about getting zerg to upgarde flyer attack or flyer carapace?

[Crispy13]

It seems that zerg ai use mutalisk when ZvsT, ZvsP, but they won't upgrade flyer attack or carapace.

I think carapace is better, because they don't micro-control mutalisk.

As games progress, mutalisks are defeated more easily by corsairs or marines, due to upgrade gap.

[jncraton]

That seems reasonable. I've attempted to speed up air upgrades (952e927). These were already researched, but not until Hive, so they were quite late.