The Mortarboard +2 does indeed enhance "grimoire: reduces spellcasting time" even further. I tried as scholar / ninja naked without and so to not have fast cast.
I used reraise because the recast was 1 minute.
used with light arts ended up with 54 second recast
used with light arts with mortarboard +1 ended with 51 recast.
with light arts and +2 mortarboard ended with 48 seconds recast