I'm still putting money on the turbos are getting to their efficiency...
The N1s are good on a stock RB26 at around 24PSi...
Now you're pushing 17PSi through...
So an absolute pressure of 31.7PSi...
Now add in the fact you're ingesting an extra 15% of that air, rather then it being a restriction...
So you're really pushing ~37PSi...
Take back off the 1 atmosphere...
You're pushing around 22/23PSi "comparison" wise...
But you have to think, at the same time, when you push 15% more air in, you're pushing MORE then 15% more air out... (Heat, expansion, yada yada yada) you're just pushing those poor turbos WAY past their efficiency range...
Get the maps out, and do the maths... Those turbos are out of their league!
A friend pushes his at 24PSi on a stock capacity 26... Makes 320AWKW... And it makes it up near 6500RPM...
You may "only" be making 300AWKW in yours... But you have to think, you're making it at a LOT lower RPM if you're running outta power at 6250... Which means, you're making more torque... So you're using more fuel, and air (Less restriction, same boost level, more air flow... ), at a lower RPM...
TURBOS ARE OUT OF EFFICIENCY... Logic says it...