Is savant-ism really ONLY attainable by flaring metals for an extended period? I would think that the more you use it, the better control you gain. Kelsier passed Vin off to everyone else to learn the nuances of their specific abilities...I always pictured that there was levels, like Pathfinder said. You get better and better as you use it, from being able to push with steel at the beginning, to being able to create a bubble. After that comes the "detrimental" aspect. You have used it for so long that it becomes the new norm, so your ability to focus the Investiture gets more advanced. Then you end up not being able to go back to the original normal, and start seeing everything in steel sight.
Same with tin. You learn how to enhance all the senses first, then focus on one, then extend the range further and further...and all of a sudden your body no longer can return to normal. Spook just took it so far that he messed himself up pretty bad. If he would have stopped pushing constantly I think he could have turned it on and off and still been considered a savant.
Just how I always pictured it. But I'm new here.