I think it's a harmless way to bring the issue to the present. Motorists regularly speed down Clinton at speeds over 30 mph. Only to get held up at the next traffic control intersection. Where I as a cyclist take my place ahead of them in the green bike box. You see, this street really isn't a race track and cyclists just want a place to ride where they wont get harassed! That, I'm afraid, will never happen because motorists have some holy righteous belief that they are the blessed children of the road. 

So, yes, 20 is plenty. Putting these signs up is nothing more than a social statement or an art installation in my mind. The fact that motorist can't tell the difference between the authentic 20 MPH sign and the 20 is plenty sign only underscores the "operational illiteracy" (the lack of a clear understanding of how to operate a motor vehicle)  possessed by the large number of motorists that rip through the streets like Paul Walker. 

If you drive politely you can ignore this statment