It sort-of works. The 8 speed will move the mech 4.8mm per click, but a 7 speed needs 5.0mm. The float in the top jockey will take up most of the eorror, until it all gets muddy and dirty.
So it works, but is tetchy and definitely not optimal. If you have a collection of garage queens that you ride only in the dry and lick clean afterwards it's fine. In the real world it is annoying.