Adult Naruto is much stronger than teen Naruto.
1 - Naruto never lost his powers from the war/VOTE battle. He is able to do everything he did during he war and battle with Sasuke. Plus he now has the full 9 tails inside his body. Not using a power/ability doesn't equal to losing a power/ability. That includes the tail beast chakra. Their chakra inside Naruto is the power that allows them to communicate to each other through Naruto. Naruto can always gain more chakra from them. The only exception may be the TSB. When Nauto fought Sasuke he had three TSB left so he atleast still have those 3 TSB. It's unknown if can create new ones to replace the ones he lost. He should be able to because RSM gives his the ability to create TSB not the other way around.
2 - Naruto has made huge improvements since the war/VOTE battle . He has even learned new jutsu/ability. Some of his improvement includes:
Kakashi Novel -
- Naruto learned how to perform handseals using one hand.
The Last -
- Base mode Naruto can create a FRS with one hand, throw it, and control it's trajectory. He can create any rasengan variation with one hand.
- Rasengan blast. That's what I call the new Rasengan he used at the end of the movie. It shoots out like a blast/wave of energy. Similar to how the beast ball can be shot out in a wave form.
- New or improve 9 tail seal. The seal now allows Naruto to release the 9 tails from his body.
- Unknown jutsu/ability he used to remove the chakra ball from Hinata's body. I believe it's yin/yang based.
- Partial chakra mode.
Gaiden -
- Wind claw.
- Healing ability that rivals the 1st hokage's healing ability. After Naruto removed Sasuke's sword from his body his wound instantly healed.
- Genjutsu. Naruto used an illusion against child shin during their last battle.
Boruot movie/manga -
- Vast improvement to base Naruto's speed, strength, and power. Base mode Naruto was able hold his own against Momoshiki while the other kages were easily defeated -
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- That was after he absorbed Kinshiki. Naruto defeated Momoshiki's strongest jutsu with a base mode rasengan.
3 - Naruto wasn't rusty nor did he lost any of his battle sense during the gaiden. What happened to Naruto was a perfect example of a character being nerfed for plot. He was nerfed because he was too powerful and the manga proves it.
When Naruto first meet child Shin he easily rescued Sarada and her friend. Then he perfected countered Shin's attack and protected the children. -
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- Naruto knew nothing about Shin.
Adult Shin tried to surprise attack Naruto and failed. Naruto saw he attack coming and easily counter it.
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Yet, after Sasuke fights Shin Naruto now performs poorly against him. That makes no sense. You can't have character performing well twice in battle and then have them perform poorly and say the reason is because they are rusty. If Naruto was rusty his first battle would have shown us that. Naruto and Sasuke should have easily defeated Shin in base mode.
Teen Naruto easily loses to adult Naruto.