Your pushing the turbo's past there efficiency rating and losing power.Those turbo's are peaking around 4200 to 4800rpm's, after that it's down hill. You will not feel it so much because by then you going really fast. Put the car on a Dyno and you will see it then. Do you have stock intercoolers? If so, your really pushing your luck, but you know that then. For that is there soul purpose, cool air so that more fits in the cylinder and power is gained. So back it down to say 17/18 psi and run 105 to 110 octane which runs cooler and your gains far out weight the path your taking.