As the 20 looses steam as RPMs increase you will see the IATs climb is what you are saying?
More boost pressure from the IS38 would cause higher compressor discharge temperatures. The fact that IAT is lower with the IS38 than on the IS20 at lower boost is an indication that the Unitronic IC is cooling better, confirming what you observed.
Peak boost is the same - the IS38 has a higher PR ceiling than the IS20 at the same psi = less heat.
What do you mean by PR ceiling, I'm not familiar with that term as you are applying it?