The thors AA splash damage has been greatly reduced. Before 2 thors could annihilate ANY NUMBER of mutas in 2, maybe 3 volleys. And i really mean any number. I lost 35 mutas to 3 thors once. But no longer. So while muta's are not the best counter vs mech because of thors. If you notice they only have 2 or 3 and you have a good sized group of mutas, go for it.
Once their thors are dead you can go for the tanks.
The other option is to try to quick snipe the tanks and ling rush the remaining thors.
If you dont understand how helpful this is, i just dont know what to tell you.
I only recently found this out on accident durring a 2v2 match and then again to confirm in the single player challenges.