I'll answer my own stupid question with details directly from the Nismo site.
Nismo big operating cylinder:
"To reduce the pedal load when using a sports clutch, improve half-clutch operating characteristics and
provide proper clutch connection positioning, the size of the operating cylinder has been increased from
3/4 inch (ø19.055mm) to 13/16 inch (ø20.54mm). This reduces pedal pressure by as much as 14%
compared to a standard part."
I'll need the 30620-RS580 for the pre Feb '93 GT-R, available locally from folks like Slide.
While I'm here, can anybody exlpain why it bites like crazy in all the gears except reverse, which is nice and smooth with plenty of notice before it bites in?