hey paul - i finally read your thread & there is a lot information & opinions in here...
my setup is similar although i have:
stock intake plenum
-10 turbos (apparently dont makes as much as power as hks items lol)
xforce dumps
hks super drager exhaust (steps from 3.25", 3.5" then 3.75" at the rear diff)
z32 afms
100mm hks exhaust
ported exhaust manifolds
3ltr bottom end (wont necessarily give you more peak power, torque yes)
...the only thing i could pick which ive raised with you in the past & DOES make a massive difference: is your head ported? this will give you up to 50awkw gain when you are playing in the 400kw range (as confirmed by worksops i use)
i personally wouldn't go changing dumps & exhaust as mine is similar & makes 405awkw (450rwkw) @ 23psi. you want more midrange so yes go with -5s BUT before u do that, pick the brain of that other dyno tuner that i suggested (name disclosed as im not here to sledge workshops).
going to smaller cams - no way! ive had this discussion with crd, red r racing & rips racing many times - unless these workshops dont know what they're talking about & smoke bongs, leave your cams in there (im running 280's @ 10.5mm lift)
work with what you have otherwise your gtr will is going to send you broke (like the most of us lol)
go forth