Chances are you'd be fine with the extra 2cm on your current post, the post just nieeds to extend past the where the top tube meets the seat tube for structural reasons... the "max" on a seatpost is a bit of a joke, and a guideline more than anything since it varies so much from bike to bike. On a bike like a Kona or Gary Fisher where the seat tube extends 2-3" ABOVE the seat tube, the "max" line on most posts isn't sufficient insertion.