this is the only problem I have concerning vs threads, please consider Itachi's fighting style not once did he resort to using his last resorts first. Anyone saying in the opening of a match any character spams x tech is doing that character injustice. Another factor favoring Sasori is his puppet was capable of flight, if I remember, making it harder to land Amaterasu.The thread had Itachi fully healthy. Itachi had low chakra because he was sick. If Itachi's not sick his chakra is perfect.
None of that matters anyway. Amaterasu is a flame that consumes everything until what it is consuming is gone, including other flames. In other words, anything that catches fire with the black flames of Amaterasu will be completely destroyed. One Amaterasu and Sasori dies.
Concerning Itachi's health I don't think him being healthy would raise his chakra at all, the only thing it would do would lessen the strain his body is under in the fight