Please take a look at my boost graph here. Notice how the boost rises and kinda starts to fall off some up top. This pull was to 6650 rpm and boost was 15.1psi.
Should I be concerned?
The belt is brand new. Just installed it.
Im running the Aster tensiontensioner bracket.

 

This is kind of what it should look like. The below graph is an amalgamation of many runs of D1SC and D1X on 380-ish to 420-ish ci motors. Neither fall off at the top. You probably already know this, but stuff to check.:

Belt slip - its always belt slip. If new belt, check alignment. I align within 1mm from crank to S/C pulley offset and 0.5mm plane-to-plane. Make sure your tensioner isn't too old. I have pretty good luck with no belt slip if I keep the alignment tight, annual belts and new tensioner every few years.
Valve instability (will show up as a sudden lean). Unlikely
Your intake piping or filter getting sucked closed.
Boost piping opening up under boost (ie partially torn silicone transition).
Bypass valve opening under boost (ie. not enough spring pressure)

There are handful of other things, but I think those are the usual suspects.

 

Im not sure how much I can tighten the belt up and still be able to get it installed.
When I do the twist test, I can only twist it about 45 degrees.
The tensioner on the aster bracket is probably the original and is on the list to get changed.

I have pressurized the plumbing and intercooler and got all.of the clamps tightened up.

I've been wondering if the intake manifold or map sensor could be leaking?

 

Perhaps a Katech manual tensioner and griptech pulley for the blower?

 

Don't forget a really dirty or inadequate air filter could cause this.