As silly as it sounds for me to say this after a lot of discussion BUT stick the S475 on and get it running and then you can see how it works because that turbo will make all the power you want
Only thing is the boost response as in the spool up will be very lazy maybe, which means torque will be less however on a track I cant see you being below 5000 rpm but I would say once that turbo gets going it will pull hard for sure.
Bigger turbos shift more air regardless of the amount of psi. Partly why you can see bigger turbos or a turbo with a bigger rear housing making more hp for the same psi as a smaller one.
The amount I know about turbos is just a hairs worth of the info available.
Tuning a turbo for specific goals is like tuning an engine with regards to cam specs, and valves lol
As regards for running twin turbos, if I had the cash I'd run 2 of those new g series 35 660 just because it would look mint
And would sound even better brapoing away because of zero contamination between exhaust pulse sound waves
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