You are correct there are not many out there are more than 2 though. There was even a built motor Greddy TT that made 636whp in a G37.

after spending hours(using that lightly, its more along the lines of months, but who is counting lol) of research for a twin turbo kit for my own car, i came to the conclusion that the best bang for buck kit would be the GTM stage 1 tt kit(for my power goals at least)

i suggest start working on supporting mods before you install a TT kit, and definitely find a good tuner to custom tune the car

good luck though :)

Also don't forget if you buy the greddy kit you probably need to purchase tune and fuel system seperatly, as it's not turn key.