He had Sabonis in Sac, who is no slouch and garners plenty of attention from the defense. Sabonis is a top 20 player by most accounts, and yet Fox's shooting numbers have been bad to average at best for most of his career. That's not a great indicator for a guy who is 27. Some of that may be him trying to carry a mediocre team and shooting too much to overcompensate for the lack of talent and coaching around him though.
However, it's tough to gauge how much better he can be in a system and organization that is considered to be top 5 versus bottom 5 in the league. I know the Spurs have taken guys like this and turned them into better shooters, fixing small things like elbow placement, release point, the way they jump and land, etc etc. The Spurs do that really well and tend to get the most out of their players, so maybe Fox will get a whole lot better. Betting on that growth isn't a bad idea, but if they are wrong it will be an expensive mistake.