Hey folks,
I did a custom Windows build with the ‘BooleanModifier.show_in_editmode’ property enabled.
The Patch is trivial, but the consequences for the editmode performance are not, ofc.
Theres a reason this isnt available for the standard build.
It might be a decision made when the much slower carve-boolean solver was still around.
Basically to protect the users from themselves. 
And maybe it wasnt re-evaluated when carve was ditched.
I dont know - lets find out!
This topic is to discuss whether it should be there or not with todays bmesh solver, so play around and report back: GraphicAll — Blender Community