Don´t know if it helps at all, but Greddy are basically "massaged" MHI turbos, mitsubishis...
I´m running a Td05h-16g from Subaru, with a 6cm2 T2 flange from an Alfa Romeo... similar spec to the T517Z... i´ve just bought 2(good) junk turbos, swapped the hotside housing and slapped that on the GTIR manifold... got 303bhp reliable from a EUDM 10:1 SR20DE, from 100usd turbos.
Garrett´s are temperamental, rare and pricey when used... where i live of course... if you going costume MHI´s are a great option...
There´s plenty of 16g´s, 18g´s, 20g´s from subarus and evos floating around, and you can buy T2/T3/T4 turbine housings from ebay, like the kinugawa ones, their made in Japan, the AUS guys love them, even full turbos...
You can buy a genuine MHI td05/td06 turbo, use the original CHRA, and choose the turbine housing, even the compressor housing for your power goals and have pretty much what you want for half the price of any Garrett turbo our brand name...
If you go to Australian Nissan forums there´s plenty of info on MHI/greddy turbos... and even more on EVO and subaru fórums...
There are even plenty of "special" turbo manufacturers that use kinugawa/kamak turbo parts, on original center cartridges...
I´m planning to use a 10cm2 T3 turbine housing on my turbo with a tubular manifold for more power... maybe use a 18g or 20g wheel/compressor on the cold side...