Stock IHI tops out at about 18 and the T3 at 20 or so, but "psi" isnt what you should be looking at. Its flow.

They just cant move enough air to make that much power, period. Sure they run at high psi but... well it doesnt really matter.

If you want 400 whp as cheap as possible the turbo youd want to use and is most commonly used is either a properly sized T3/T4 hybrid or a Holset (HY30 or HY35) with bobs log header and either his gutted/ported intake or his custom intake, and get your head ported or get an esslinger head.

That said, a guy who home-ported his iron head (the aluminum esslinger head will outflow ANY iron head by the way the passages are cast...) with a holset and a rotated gutted intake (shorter piping) and bobs log got 430 whp with great spool. Its a wonderful engine and cheap as hell, just dont rev it high on old rod bolts!