Originally Posted by JasonC SBB

You need to look at the % drop in torque from 5500 RPM to 7000 RPM.

On my GT2554 it was like 10-12%, when at 10 psi.
On Ian's GT2560 at 12 psi, it was the same.
I'll bet on your GT2871 with the stock IM at 15 psi it's the same.

On my GT2554 at 13 psi the drop increased to close to 20%.
On Ian's GT2560 at 15 psi the drop increased to 20% also.
Ditto for your setup with stock IM at 21 psi.

Do you see a pattern? The turbine is probably choking.

Note that the setups that choke out appear to max out and flatten at a given power as opposed to continuing to climb.

And of course, switching the IM out to an IM that breathes better at the top-end will unchoke the turbine, because the compressor needs less shaft power, good for another 3 psi or so. My theory will be supported if the data shows that your setup chokes at the top with another 3 psi, and if other setups that show the large torque drop at the top-end, show rocketing TIP.