The JUN plenum is the only one I have seen (and I've seen a lot) that is designed properly - as for the GReddy plenum; it is nothing more than a toy.
HKS GT-SS are capable of making around ~350 AWKW, but that's at the extreme end, and Jim obviously tuned your car for reliability and streetability - which sounds like what you'd asked for.
HKS GT-RS are capable of somewhere around the ~410 AWKW mark.
Note that there are a myriad of variables here, things like cams and their dial-in, valve lift, exhaust manifold design, intercooler, fuel pressure, tyre size and pressure, and much more affect your final power figure, so don't take the above figures as gospel.
Mario.