it usually works out pretty close to power=torque x rpm / 10,000
 
so if your making like 350nm at 4000 rpm should be round 140kw
or 350nm @ 8000rpm=280kw  
at 4000rpm (120kph depending on tires & speedo) you're making 170kw on 20psi .... which should be your peak torque too.
 
i'm guessing you make peak torque of around 420Nm.
if your torque stays flat that should mean round 255Nm at 6000RPM (170kph) which is about where you're at, then the torque usually starts to fall off as the turbo runs out or the head/cam/etc. becomes too restrictive.... the revs go up but the torque falls and power kind of flat lines....which is what you're seeing.
 
shame i can't get my graph to show, I'm only running 14psi, but round 6100rpm, where the torque starts dropping we ramped the boost to 20psi at redline to keep the torque flat.... consequently my power keeps going up.
 
it's kind of cheating, and it means i hit redline a lot, well that and jumping off the bike which screams to about 16,000rpm, sometimes i just forget to change ;-)
 
not giving you a lesson Dyl as i'm sure you, and many others, know all this, but figured a bit of an explanation might be helpful to people befuddled by their first dyno chart or working out how to read it and work out where to go next.
 
the fact that you're managing to hold 20psi till the end of the chart shows the turbos still got legs at that poist for that pressure, wheras a CT26 would have maxed and couldn't flow enough to keep boost up, and a gt3071 would probably have peaked also and be on the way down..... if your boost is holding, but your torque drops then you're either not getting enough air or fuel. If your boost drops then you need to do something about the turbo.... or just accept that you're not going to get more power up top and not bother with mods aimed at the high rev range.