I don't think this is a useful distinction. First, because on-ball and off-ball isn't really something that gets parsed out statistically. You can have assisted versus unassisted, but that isn't the same thing. Beyond that, you don't really get a percentage for assisted attempts, since by definition, every assisted attempt is a conversion.
I would assume, like everyone not named Jimmer Fredette, that Manu shoots better "off the ball" than he shoots "on it". But the second reason why he's not a useful distinction is that not all off-ball threes are created equal. Bowen's three came off the ball, and so do Korver's. But Bruce was much less dynamic, since he only shot from certain spots and wasn't really a guy who affected a defense with his movement around the perimeter. I don't think there's an easy way to get stats on this either, but I don't think Manu is very dynamic off the ball. I have watched some highlights recently and do see him scoring on some shallow cuts and screens, so he's not completely stationary. But he's not really putting pressure on the D with his shots or attempts.
If Manu is being aggressive with his shot, those will come up "on the ball". But for the most part, his threes are reactive. They come off someone doubling another player or overhelping. In that regard, what separates him from Tony, Pau or LMA is simply increased opportunities and willingness to shoot. On a given play, his role is not to force rotations by cutting across the formation while someone else does something. His role is usually to be that someone doing something.
I don't want him to be another Kerr or Horry. Those guys are from a different era. Their performances aren't nearly dynamic enough to work today. They'd have to play in a different way -- at least Kerr would. The game is just different. Manu as an off-ball player can't sit back and do that, because Bertans could come in and do it sooooo much better.