Debatable, but I guess it is how you define "sleeper" Look at 2009 so you have some history/ data on players. Curry was 7 (as much of a sleeper as Lillard was at 6), Derozan 9, Jrue 17, Teague 19, Taj Gibson 26. All preceded by far inferior players. No way to predict NBA success, but I agree scouting and analytics have improved team's ability to predict.
2010 produced 40th pick Lance Stephenson....
This.
There will always be guys who are "hungry" enough to get better and thrive in their new environment. For them, the NBA isn't the goal, it's the start line.