The Subaru WRX differential is an R160 which is smaller than the stock 240Z R180 (160mm ring gear vs 180mm ring gear). The R160 was orignally found in the 510's. The R160 will suposedly fit in a Z, but I don't know how well it will hold up. An R200 LSD (200mm ring gear) is your best bet for durability.
If you're talking about just the limited slip assembly from an R160 bolting to an R200 ring gear, I haven't tried it personally, but I'm pretty sure the R160 assembly is proportionally smaller.
i think the weakest point in all of our diffs are the spider gears. from what i know open diffs have 2 spider gears and lsd's have 4 spider gears, so even though its a r160 it has 4 strong spider gears