I'm not sure I like this extra 'effect'. My initial reaction was that it was pretty cool, but thinking about it feels like Sanderson gives himself 256 ways to sort of legitimately use a deus ex machina. Make that 257 with Harmony and his willingness to hand-wave some extra guns when the hero needs them (thinkin AoL climax here).
Shooting bullets out of the air (even if the bullet in question was hanging still) is pretty amazing, yes. I disagree with how fast he's learning, though. The SoS proloque is 17 years before the first chapter of SoS. SoS is about a year after AoL (Steris remarks they know each other about a year) and IIRC AoL is mostly set about a year after Wax leaving the Roughs (I think ~6 months between first chapter and prologue and ~6 months between first chapter and meeting Steris).
That leaves 15 years of becoming a proficient gunshot. 15 years in the Roughs, hunting criminals, that is. I'd count that as full time, highly motivated training. The end result is very impressive, but not incredibly fast.