you shouldn't have been hitting 17 before the ap. after the ap yes you should be hitting 17. the stage 2 map is not designed for headers. that is the piece that controls your boost. the only leak that will cause a boost fluctuation is a leak before the turbo. you are not tuned to perfect, in fact it's a little far from perfect. yes you will notice a big difference in the stage 2.
my advice: replace the gasket to maybe get a little less noise. then get a tune. you are after firing at idle because you are running rich(all off the shelf maps are rich). your boost should be smoothed out and good. some extra noise will be there.