If anyone had doubts about his skill they just need to read any mention of him in the manga. Everyone knew just how much of genius he was. He created 2 of the best techniques seen in the manga (Rasengan and FTG) and developed the theory for Naruto's Wind techs which he probably could have perfected if he lived longer. And he did everything without super high chakra reserve, bloodline limits, or a tailed beast.