I wasn't impressed with the HS heads. Had a 2200, several 2300s and a couple HS.  They changed the shape of the floor, and I couldn't seem to cut as fast.  Also didn't like the segmented reel tine bars, always coming loose somewhere.  My cam track wore through several places.  Was happy to see my last NH (HW325) leave the yard 2 years ago.  Agco WR9770 now.  The 2550 was the best machine NH made in my opinion.