It's a big if, but if Kelly is able to create an effective offense in SF and the team doesn't quit on him, then I think this move would be a net positive for Seattle. The Seahawks have had a lot of success against teams using spread concepts relative to the rest of the league.
Arizona, on the other hand, has showed some vulnerability against top end spread offenses, as evidenced by the two games against Seattle this year and the 511 yards they gave up to Chip Kelly in 2014. Arizona did blow the doors off against Philly this season, but it's legit to wonder if the Eagles had quit on their coach by the time that game was played.
It will be fun to see Kelly in a division that is shifting more towards an emphasis on offense. If he can build something quickly like he did in Philly then it will be fun seeing more shootout games.