There's no evidence he's better at finishing than Cedi or Okoro. I'm afraid I'm going to have to have you show your work on this one.
Fair enough. Last year (large sample) shooting percentages at the rim per cleaningtheglass.com:
Stevens 69%, Okoro 63%, Cedi 62%
This year (small sample):
Cedi 74%, Okoro 62%, Stevens 60%
I don't know if they factor in getting fouled at the rim since that is not a shot attempt, but it does result in free throws. Also, sometimes you get an uncontested dunk off a pass following dribble penetration, so a better number would be something like points scored (both on buckets and free throws) per contested shot at the rim. But I don't think they keep track of that.
I thought Stevens was the best of the three at finishing contested shots at the rim, but I don't have the numbers. At this point they're all pretty good for wings. Cedi has improved at finishing since last season. Stevens is down from last year, but it's a small sample.
I think the key is effective shooting percentage, which takes all shot attempts into account, not just at the rim. This year Cedi is at 60.3% (83rd percentile for forwards), Stevens at 58.3%, and Okoro at 42% (5th percentile).
Cedi's eFG the last three years (including this year): 45.8%, 54.6%, 60.3%. His shooting percentages at the rim: 55%, 62%, 74%. In his 6th season he still seems to be improving.