I've posted this several other times this offseason, but it's worth repeating:
Here are the top receivers in the league right now and how they rate in terms of BMI/40/vertical/broad jump.
Andre Johnson - 29.5 (4.41 - 39" - 11'0")
Vincent Jackson - 28.9 (4.46 - 39" - 10'9")Dez Bryant - 28.8 (4.52 - 38" - 11'1")Victor Cruz - 28.3 (4.47 - 41.5" - 10'5")Calvin Johnson - 28.3 (4.35 - 42.5" - 11'7")Larry Fitzgerald - 28.3 - results not availableHakeem Nicks - 28.3 (4.51, 36", ??)Michael Crabtree - 28.1 - did not work out before the draft
Demaryius Thomas - 27.8 - did not work out before the draftJulio Jones - 27.8 (4.34 - 38.5" - 11'3")Brandon Marshall - 27.6 (4.52 - 37" - 10'0")Roddy White - 27.2 (4.46 - 41" - 10'6")Steve Smith - 27.2 (4.41 - 38.5" - 10'1")
Reggie Wayne - 26.9 (4.45 - 36" - ???)Marques Colston - 26.9 (4.50 - 37" - 10'3")Percy Harvin - 26.7 (4.39 - 37.5" - 10'1")AJ Green - 26.0 (4.48 - 34.5" - 10'6")
This paints a pretty clear picture. With few exceptions, the situation-proof superstar NFL WRs test very well in workouts. They have size/speed/explosiveness that show up in the drills. Every player on this list had at least a 10' broad jump. All but one had at least a 36" vertical. 4.52 was the slowest 40 time. The averages for all of those drills are significantly better. It's also worth pointing out that the 28-29 BMI range is the most well represented in this sample. In other words, heavy receivers dominate.
People obsess over production and skills, but the list above suggest that a receiver's raw physical talent plays a big role in determining his upside. You just don't see a lot of mediocre athletes becoming superstars at the WR position. There have been a few notable exceptions like Anquan Boldin and Chad Johnson in the past decade, but even Boldin had elite size with his 28.9 BMI and Johnson was a guy who played far more explosive than he tested. Even if you include those guys, they represent a small minority of the total elite #1 type of receivers in the game.
This is not to say that measurables are the only thing that matters or that every workout warrior is going to be superstar. Don't confuse necessary for sufficient. You have to be tall to play center in the NBA. Not everyone who's tall can play center in the NBA though. Likewise, you have to have special physical qualities to be a high end #1 WR in the NFL. However, not every WR prospect who tests well is going to become a great pro. I wouldn't take a UDFA like Mark Harrison over a high 2nd round pick like Robert Woods just because he tested better.
However, in the case of two players with a similar draft position and outlook, I'll usually favor the elite athlete over the modestly talented overachiever. Boyce and Bailey were only picked 10 spots apart. The difference is so minor that it's pretty much negligible. Here's how they stack up physically using the numbers from above:
Stedman Bailey - 27.5 (4.46 - 34.5" - 9'9")
Josh Boyce - 28.6 (4.34 - 34" - 10'11")
Boyce is a much more gifted athlete. Bailey has pretty good weight for his height and his 40 time is solid, but it's worth pointing out that NFL.com has his combine time as a 4.52 (and Boyce at a 4.38). For whatever reason Draft Scout's 40 times aren't consistent with NFL.com's numbers this year. You can decide for yourself who to believe. Either way, Boyce is significantly faster. Their verticals are both disappointing. Bailey is better, but only by half an inch. Boyce blew him away in the long jump, with an elite 10'11" mark compared to a below average 9'9" jump. Boyce is also bigger and heavier. I don't emphasize bench press and the three cone drill quite as much, but Boyce trumped Bailey in those drills as well. He did twice as many reps on the bench. He had the 3rd best WR time in the three cone drill compared to the 8th best for Bailey. Overall, Boyce ranked in the top 5 at his position in 5 of 7 drills at the combine. Bailey ranked in the top 5 in exactly none. So you're talking about a borderline freakish athlete vs. a completely pedestrian athlete.
Having said all that, there are numerous successful NFL WRs who aren't incredible workout warriors. Guys like Randall Cobb, Jordy Nelson, Eric Decker, Wes Welker, Lance Moore, Steve Johnson, and Mike Williams come to mind. Personally, I don't think any of these guys are situation-proof elite #1 NFL receivers though. They're good receivers who have thrived when thrust into favorable situations. Johnson has benefited from Buffalo's lack of viable alternatives at WR (he gets a lot of targets by default). Guys like Decker, Nelson, Moore, and Welker have only been relevant with elite quarterbacks passing them the ball. I think this is about the best case scenario for Bailey. He will never be a superstar in the NFL, but if he pans out and the stars align, he can potentially become a top 20-30 WR in FF. That's his realistic best case scenario though. With Bradford looking pretty mediocre and Austin/Givens already on the roster to siphon targets, I don't think this situation looks promising enough to elevate an average talent like Bailey to weekly relevance.
Boyce has warts of his own. He's not quite as tall as you'd like, he doesn't have a great catch radius, and his vertical leap is a bit lower than you'd like to see. He's not an elite prospect either, but he has rare athletic tools with his 28.6 BMI/4.3 speed/10'11" broad jump/6.68s three cone time. That package of speed/strength/quickness is going to make him a tough cover in the NFL. He might not ever become a superstar either, but he seems to have a much higher ceiling than someone like Bailey. He also goes to a better team with a thin WR depth chart and a HoF lock at QB. To me, this is an easy decision.