Sai is not the strongest, he just paints and hopes his cats and birds hit the enemy.. Neji simple spinning around would stop all his attacks..

Sai was an elite member of Root, Danzou believed strong enough to kill Sasuke, he's also been trained as a weapon since a young age, so i'd think he would be atleast an even match for Neji.
he grand slammed neji because of the kyuubi power without it he would lose.( he win because he had two chakras)
sakura is not faster so stop sayin this
poison and bombs the strong people in the manga are immune to poison like sasuke
No He managed to overcome the blocked Chakra coils with the use of Kyuubi's chakra, he beat Neji by using his brain, taking advantage of Neji relying on his eyes to distract him and beating him. If you make such a claim then you can argue Neji is only good because of his Bloodline, he would have lost without his abilty to see chakra veins to block Naruto's coils.
Sakura's speed and agility was proven in her battle against Sasori, she was able to avoid the being attacked from multiple angles, She was trained personally by Tsunade, and as mentioned the official databook puts her ahead of Hinata, We've not seen anything that suggests Hinata is faster, yeah she was in the first part, but we've seen nothing to suggest that after the timeskip.
You also mention about her staying back and letting her teammates do the hard work? She was fighting Sasori, head-on, she destroyed than summon from Pain head on, she was fighting with Lee, Kakashi, and Gai, as a close combat specialist head on. Besides she's one of the high ranked medics, protege of Tsunade, with the potential to surpass even her shisho, she has to avoid getting severely injured, she gets injured in battle, means the other can't fight to their full potential because they don't have anyone on the battlefield to heal them. Her role is ultimately to support the others so they can fight all out knowing if somethin happened they have someone who can heal them and they can fight on.