400 rwhp is a big ask from the HKS kit. Also the additional mods is nonsense, its only if you use the base map that comes with the S-con thats included in the kit.
I was running Cosworth Plenum, DC headers, Tomei decats, custom y-pie & a greddy exhaust with my HKS kit and 380 @ the hubs at 7.5psi on Abbeys dyno.
There is a guy on my350z that has broken the 400whp barrier but he was also using aftermarket pulleys & the stage 5 GTM kit aswell as similar mods to what I run