How many times have you actually paid attention to how fast a kicker runs or how many times he can bench 225? My guess would be never. There are several big athletic kickers in this league, but they kick (Jeff Reed, Sebastian Jankowski, Joe Nedney). Just because they look like they can hold their own in a bar fight doesn't say they can play special teams football at a high level.
I know it is all exciting to pretend we have this jack of all trades Superman who can kickoff, kick long field goals, be an up-back, play on punt coverages, cure cancer and so on. But until I see this player actually doing any of the things that are being bandied about in this thread, I won't take him seriously as anything else but a specialist. Just because USC experimented with him at fullback and safety in practice doesn't mean he can come close to performing either duty at an NFL level.
People want to take this player and make him an up-back. We had bigger stronger and more athletic players playing on our punt teams and McBriar got injured because one of them didn't do his job. But since he's a "freak" that can run fast and bench a lot, he's suddenly qualified to do more? He's a project until I hear DeCamillis say he can do it. So far, all there is nothing but conjecture and some overactive imaginations based off fluff generated to explain why on earth we drafted a kickoff specialist in the fifth round.
The real discussion should center on the impact of carrying two specialists on the roster. I don't think there is much evidence to support the idea he's most certainly going to do a lot more. Let's see him in action doing these things in preseason before we annoint him.
He'll do a good job as a specialist. The issue is at what expense.