I think the C177 cardinal tailplane slot was a design mod. added at some point after certification when problems were identified with the implementation.
I recall reading - somewhere, damned if I know where - that there were incidences of tailplane stall during landing.
The hold off with increasing AoA of the tailplane eventually exceeded its ability, resulting in a stall ==> loss of downforce ==> slam the nosewheel into the runway.
The whole thing seems to have been an attempt to cater to fashion ie "Our competitor is selling a whole series with it & beating us over the head with its theoretical advantages....so we'll have one too."