The only thing that improved Warner's fumbling were the gloves. Once he could hang onto the ball, he was back to his MVP self. I know Warner got the idea to try them from watching Roethlisberger but I don't know if that was at Whisenhunt's urging. The dots are there to connect but I don't think Whisenhunt coached the fumbling out of Warner.
Warner broke his thumb as a Ram and it never healed right and he lost strength and/or dexterity and the ability to securely grip the ball. From '02 to '06 it was the same thing for him. He could still throw and read a defense but he couldn't stop fumbling. On snaps, when he got hit, even when he didn't get hit. As a Ram, a Giant and as a Cardinal. He lost his job 3 different times because of the inexplicable fumbles. When he mopped up the 15th game of '06 and started the last game of '06 when Leinart got hurt, he'd adopted the gloves and he was instantly back.
Many don't put much credence into the gloves theory but it seems very obvious to me. Those tacky, diamond handler gloves kept the ball from squirting out of his hands and he was able to stay on the field. None of that was Whisenhunt. But Whisenhunt does deserve credit for playing the best player. Not all coaches would have gone with the washed-up vet over the first round draft pick.