Bradley had 14.1 shots a game last year. If Hayward got that at a similar percentage as last year, he would score 19.54 ppg. I can't see how Hayward doesn't get at least 14 shots a game on the team with the system. He might not score quite effectively but there is no way he isn't at least second on the team in shots or points.
Yes, and if he can't replicate his efficiency from last year, and is more like the year before instead, he'll score more like 18.5. And that's purely a mathematical projection assuming he's completely unaffected by playing with Kyrie Irving and his 30% usage rate. So I see 18-19 ppg as a realistic scenario, and 20-25 as something that is probably not happening.
It's also worth pointing out that on a per-36 basis, Hayward averaged 14 pts (on 14.5 shots) in preseason play, and Horford 17. But then again, Smart averaged 20+, so there's that