The subject line asks it all, 300ZX rears look pretty good, why aren't they being used as an upgrade to earlier Z's?

Probably because there are better conversions around?

Or, no one has taken the time to make the mounting plate to mount the calipers. But, the rotors might not have the right offset so that a mount could be made to use the calipers...... Good question....

Might have to go find one and find out why.... That is unless someone already knows. With all of the mixing and matching that goes on now surely there is a combo that would work with offset, or at least you would think so.

As far as better upgrades... well there are upgrades but a steel caliper weighs more than aluminum so maybe better is yet to be found...

The main trouble with the aluminum 300 caliper is they were only offered for a short time and their supply is kinda limited compared to the later model. I've bid a couple times on the aluminum ones on Ebay but backed off when the prices went up in the 300-400 categories, for used ones needing to be rebuilt..... there's a lot of interest in them and they do command pretty high prices. That might be the biggest drawback, as you can do the 200SX complete kit for what the aluminum calipers cost.