Sai would down Neji easily.
1. Danzo said Sai was the strongest of his age group.
2. Yamato admitted that the people in Sai's bingo book he had taken out were no ordinary shinobis.
3.Also during their training when he had a fight with naruto and a naruto clone transformed to captain Yamato to trick him and gave him a blow hee warned Naruto that he should never think he is stronger and Naruto admitted that.
With the above reasons I think Sai will easily own Neji