One thing to add.
Speed factor is not measured solely by 40 time. There are a number of measurements to give you an overall idea of what kind of speed a guy has, as defined by his ability to transport himself quickly.
It's just like in horse racing when they give a horse a speed factor. It isn't just a straight up measurement of how fast the horse runs like 6 or 7 furlongs.
In this case you measure a guy's 10 yard split, his 20 yard split, going on up all the way to 100 yards. There's 3-cone stuff, shuttle runs, and other speed tests. From all accounts he should be stellar in all of them.
The scary part about Bloom's speed is that when he reaches the 40 yard mark to cap off something like a 4.29 electronically timed 40, he probably has not finished accelerating yet...he's probably just reaching 5th gear. Get that into the open field, add in the agility, coordination, and vision you need to be a world cup gold medal winning moguls champ, and the guy will be flat out exciting.
You know what is fun? He's got the next 3 or 4 years to learn whatever he needs to know about route running, punt returning, and kick returning. In the meantime all you really need do is tell him where to run and it's like Forest Gump on the Crimson Tide.