While I tend to agree that good prospects produce at the AHL level, it's not fair to compare Veleno to Chytil who is 21 and has 2 years on him. If you look at the past 20 years of production in the AHL for U20 players a player with a point per game above 1 usually leads to a superstar level forward, 0.8-0.99 looks to be at around a top 6 forward level, 0.5-0.79 a middle 6 role, and less than that it's either a bottom 6 or a AAAA player. Veleno has started slowly, though I'm sure he can pick up the pace and end the year in at least the 0.50 range given his previous levels of production.
Zadina on the other hand will need to pick up his production to maintain his status as a true top prospect. Almost all top 6 players score at a > than 0.75 p/gp in their U21 season, anything less than that from him would be a disappointment. Given his production last year and the pick used to select him I think everyone has high expectation for him this season.