The way I'm thinking, I'm not talking about just one stop. Planes for example have small setups for what they do. They are only good enough for one stop then over heat and have hours or even a day to cool before they are needed again. With a 280ZX with a huge power plant, after multiple high speed stops, you will need some decent hardware. For city driving it should be fine, but say you were to go along the Great Ocean road, you will find their limits. Plus changing tyres puts a bit more stress on brakes. Going to a semi slick from a road tyre offering more grip, will make it harder to lock up.
To me, the faster you go, the better you have to stop!
One thing you are forgetting about the advantage of going to bigger brakes is the heat. Like you said above, only after the upgrade, only having to press 50% on the pedal, you decreased pedal effort and heat in the brakes. Both of these very favorable! It's upto the owner what he wants to do first... I'm just recommending what I would do first..