That's a very odd hole. My MC didn't have it:
Z32 MC info
I say, tap the hole, use some thread sealant and a screw. Or thread lock.
As for the feel, it's awesome. I did all 4 corners first, and the pedal was super soft. Very unsettling. I just pulled the cover off of my 240 and drove it last night. I had installed some Porterfield pads up front, last fall, and never really got a chance to try them out before I put the car up for the winter. After seating the pads a bit, I was stunned by how powerful the brakes were and the feel of the pedal. The Hawk HPS pads I had before just sucked in a huge way.
-Alex S.