There's been some, I guess, rumors, that olympic lifters can actually out perform olympic sprinters in the first 10 or so meters of a run. But that's mostly hearsay, so grain of salt. But if it were true, Usain could maybe increase his speed to around 30 mph, seeing as how his workouts seem to consist of mostly high rep and explosive movements, again, if it's true. There has been talk of the human potential to hit 40 mph, though, so 30 is atleast do-able, maybe.

Training for that would be difficult, might require some borderline nonsensical equipment, like an underwater treadmill with a harness attatched to elastic band that would pull the user down to act as gravity, then let the user run against the water for a unique form of resistance training. Or just go the Baki route and fill a pool with maple syrup and run against that.