i read in Boards mag they loved the shox. it said something about every tester would rather sail with one than with out and that it did help the speed a lot... also the north team have won every uk BSA slalom even this year using the shox extensions so they must be fairly good.
i hear most of the north dealers have demo extension available which is what i will do to really test them for myself. i gather you should use a gps to really see how much faster they are when you go from a normal extension to the shox then back again.
i look forwards to hearing more about this.