I have heard there is a new gearbox that is recommended for the Z: Tremec T56 Magnum. https://malwoodauto.com.au/product/tremec-t56-magnum-6-speed/
T56 isn't that new, and it requires a fair bit of work to get it to fit. This path has been traveled. Where did you see it being recommended? It certainly isn't a bolt-in affair.
A person on facebook, of course, he also said they were trying to get a Z fitting kit. I thought the magnum was a new upgraded version of the old t56?
It's pretty rare to need to upgrade a gearbox in the Z, the T56 is like the LS of manual gearboxes though, heaps of options and goes in anything with enough work.
Nah, it doesn't actually have anything to do with the standard T56, it's a version of the 6060 box, but for the aftermarket...market. Look they are a bulletproof box, and fast, and with some fabrication you could get it in there and parts and support are numerous. It'll just come down to pain vs pay-off. Zed box's aren't hard to come by yet and they're pretty cheap, and unless you're chasing some fuel economy the 5-speed does a good job.