The 300ZX turbo has slightly higher compression than the 280ZX turbo so I would guess that is one reason the 300 timing is farther back.
Also the 280ZX ECU is not very precise on the fuel map and runs a bit rich under any sort of boost or anything above 3000 rpm. The richness and low compression fights pre-detonation. (also meant to run low octane fuel) 300zx runs more lean but it is a more proper mixture for more torque and horsepower.
If you're running high octane fuel only then it would make sense that you can run the advanced ignition timing. Listen for pinging though. Also, are you at stock boost right now? I would assume so since you made no mention of other modifications.
I did read that 1988-89 300ZX had a smaller turbo, lower boost, but higher static compression. The ECU supposively pushes timing another 2 deg advanced under boost. You won't have to worry about this if your ECU is truely a 1987 though. 8-bit