This sort of comes around to the same question....
....what do want 500rwkw for? and if you really want 500rwkw then it's not going to be anywhere near responsive.
I know 500 is the new 400 which was the new 300 in the 90's.
If your looking for a quick street car, especially something you can punt hard up the west coast, then start thinking 350rwkw max.... a responsive 350rwkw gtr will be faster almost everywhere that a 500rwkw GTR! then by the time the 500rwkw GTR starts becoming faster, your well and truly over any speed limit in Tas.
My suggestion is to have a think about what you want to use your GTR for, then whatever your choice, build the car for it... not just the engine and be happy with the comprimise.
You'll never find a turbo combo that is responsive enough or produce enough power...
...just build something that puts a smile on your dial!
Cheers
J.