For the record, Kam has run as fast as a 4.47 in college before the hip injuries, which was right about the time he moved from CB to Safety. I'd also say that he's also one of the more quick twitch safeties in the entire NFL. The consistent "flash" through your screen is a tell, but I think where his CB feet and explosion were really on display was that SB coverage of Welker. To be able to sink his hips and come off that route and get to where he needed to be was uncanny.
That said, we have no idea how fast Kam is right now especially after the hip surgeries and other relative health. He's likely a good solid 4.5, which was plenty fast enough to chase down Krapernick 65 yards downfield even after being cut blocked.
On that same token, we also don't know where Bailey is at in that department. He very well could have broken that 4.6 barrier by now. We'll see how fast he plays onthe field Sunday. Some players are a little faster when chasing and the predator kicks in. I know it certainly works that way for Kam.