Megatron was part of an 0-16 team, and a 2-14 team.
Dez Bryant was drafted in 2010 after a 11-5 season that ended after a loss in the 2nd round of the playoffs. After the draft, the Cowboys have failed to reach the playoffs or even have a winning season up until last year.
Julio Jones has shown no real evidence of influencing the win/loss record. Prior to Julio, the Falcons were 13-3. In Julio's rookie year, they had a record of 10-6, followed by another 13-3 record. The following season Julio was injured and they dropped to 4-12, but the year after (last season), with Julio playing in 15 games, the falcons finished 6-10
Look at last year's rookie wr class (Watkins, ODB, Evans, Benjamin, Cooks, Landry, Matthews) for all the hype they received, only Watkins' team improved and most would say that was a result of an improved defense. Every other top rookie WR's team record was worse than the previous year except Matthews and Landry (both teams finished with the same record as the prior year).
I'll give you AJ Green. The Bengals went from 4-12 before Green and 9-7 plus a playoff appearance in Green's rookie year. They've been in the playoffs every year since he's been drafted. However, they brought in AJ Green and Dalton in the same year, so it's hard to determine who had the biggest hand in the success, but i'm willing to give Green the benefit of the doubt.
With all this being said, I can't find any evidence of a DT greatly affecting the win/loss column either, but I can't help to think of teams like the Seahawks, Patriots, Ravens, 49ers who were able to achieve success with no superstar wrs. If Leonard is there at 4 it would be a mistake to pass on him for a wr. Honestly, at this point, If Leonard is gone, I'd rather take a chance on another defensive player or trade back and grab a wr in the 2nd rd.