To me Benjamin looks like an undersized tightend. If he would have ripped off a 4.4 flat, I'd be intrigued. But a 4.53 is nothing more than slightly above average for a guy his size.
Does anybody remember Roberto Wallace? He was 6'4" and 225lbs. He ran a 4.49 40 yard dash.
Personally, if a receiver is 6'3" or taller, they had better be able to run at least a low 4.4 or better. Receivers are supposed to be fast. If they're not fast, or extremely technically sound in their routes (which I'm guessing this kid is not), they fail to create separataion and wind up like Wallace.
A prime example of a large receiver that is fast is Demaryius Thomas for Denver. He's 6'3" 236 and ran a 4.38. He went 22nd overall. Thomas was a raw route runner because he came from Georgia Tech. But because of his size and speed he was able to develop nicely.
Benjamin is already behind the curve because he lacks anything elite about himself other than his dimensions. His speed is mediocre. His route running is mediocre. If this kid were a 2nd or 3rd round pick, I'd say go for it (if Hickey is a best player available type GM). He reminds me a lot of Alshon Jeffrey who I wanted the Phins to take in the 2nd round a couple of years ago. But we have so many bigger needs right now I think it would be a huge gamble.
Brandon Marshall is not a blazer at all along with Jeffery, and he is one of the bigger and better receivers in the league along with Jeffery. It get's put out there a lot, but Jerry Rice was a 4.6 40 guy. Dez Bryant is a 4.52 guy, Larry Fitzgerald ran 4.63.
I personally probably would not take Kelvin at #19, because I think he still needs to develop. But he is a talented kid that would be a nice get in the 2nd round. And also, his numbers would have been a lot bigger if he did not have 2 other receivers on the field with over 900 yards in reception yardage.
Just for reference, Marshall (4.52) and Jeffrey (4.48) are both much faster than Benjamin (4.61 official 40 time). In addition, I seriously doubt that Benjamin is as aggressive or physical with the ball in his hands as Marshall. But I do agree 100% that if that if the kid is there in the 2nd you take a serious look. Simply from a best player available standpoint he looks the part of a much slower Demaryius Thomas, which is appealing. If the Dolphins are going to look at an offensive weapon with that first pick, the guy you want is Eric Ebron. If Dustin Keller isn't able to return from injury as a cheap free agent option, then Ebron is a nice fit. Ebron would give us a huge weapon that could threaten all parts of the field from the tightend position. This would allow Clay to go back to playing the H-Back role we initially drafted him to play. Can you imagine Tannehill having Wallace, Hartline, Gibson, Ebron, and Clay on the field at the same time?