Flipping a 6 degree stem will make quite a bit of difference in the bar height and lengthen the reach. I don't agree that you should need to be moving around on the saddle, if all else is adjusted correctly. Lowering the bars by 2cm may require a small change in the saddle angle to bring the nose down a bit, but try it without, first. Lower only if you feel excessive pressure at the front. Some posts with serrated angle adjustment work poorly. I always use a 2-bolt rocker style post, like the fsa k-force.