I think it's down to Devin Smith and Austin for the 6th receiver spot. The top 3 are locked in, Cedrick Wilson is 23 and offers legit 3 level receiver potential down the road, and Noah Brown offers skills as a blocker and gunner that the speed guys don't. Austin and Smith are looking to play the same role.
As far as Johnson - there are a million 4.3 sprinters on the fringes of the NFL. Speed is overrated for WRs IMO; obviously you have to be able to be able to run with 4.4 CBs to threaten them deep, but you also have to release cleanly off the line to take advantage of that speed, and then be able to track the ball in the air, and then be able to adjust your throttle to get to an overthrown or underthrown ball, and then be able to fight off a DB for what are usually contested catch situations 40 yards downfield. Deep passes are low percentage plays, and they take a heck of a lot more nuance on the WR's end than simply running fast.
From what I've seen, tracking the ball in the air is usually what separates the deep WRs who make it from the deep WRs who don't. Tavon Austin, for example, is probably a sub-4.4 player right now with instant acceleration and joystick agility, but he's never really made plays as a deep threat unless the defense leaves him wide open, because there's nothing more to his receiving game than running fast. I love his ability with the ball in his hands, but he has always sucked before the catch.
Devin Smith, OTOH, was a guy who could both blow the top off the defense and adjust to errant passes at Ohio State, so he offers a lot of potential if his knees are OK. My only question is does he have enough juice left to threaten starting CBs - it seems like the plays he has been making both in camp and in preseason games have all come against deep-stringers.