The stock 240Z brakes are fine for street use, and adequate for limited track use. For track use, you need to bleed the brakes with DOT4 fluid before each event. You also need to adjust the rear shoes before each event too. I have no problems with fade during 20 minute road course sessions. I run Porterfield R4S pads, which are great, but squeak a lot when cold. Of course ITS cars are requied to run stock brakes (not pads), so that tells you something about the stock brakes.
I ran the 4x4 calipers for one season, but didn't really notice much of a difference in stopping power. No difference in fade resistance. You need vented rotors to really notice a difference there.
I am however upgrading to a Wilwood setup this winter. You can only go as fast as your brakes are good (if that makes any sense).
Pete