reducing the stem shank is one of the little tricks that add fpe but it works best when you do the full package of mods starting with flow into the valve followed by volume of the valve followed by reducing the valve spring tension (i tend to skip this bit as it can add to hammer bounce)
you then have gas flowing the stem and reducing the stem where it sits in the valve exhaust follwed by angling the port to 20*(works best if your making a valve from scratch)
you then get tp increase to 4.2mm with better seal(i like to mill the tp seat flat and use a gasket top and bottom like the p-rod pistol) followed by barrel port to 4.2mm(.22)