Who's to say that the least productive part would be cut or not? It might be that those useless meetings persist, and the actual work goes down. Certainly spreading the work out so much increases friction when it comes to collaboration. You'll often find yourself waiting at least like 4 days to get questions answered.
By "least productive 20%" I also meant the time during the week when your oomph is gone. IIRC most experiments on shortened workweeks show that motivation and productivity in general go up because workers are less stressed, better rested, happier, etc., and output does not take a big hit.
Of course it might be just an observation effect, but I think that is normally controlled for.