What gain settings you using on your blitz controllers?
If you have it too high it can knacker the actuator and cause the boost to tail off quicker.. Think you could have a dodgey MAF sensor too with only 245Bhp

I have my Gain@15 & Ratio@72 for channel 4 holds 23psi to the redline in 5th gear. 3rd & 4th gear peaks 23psi & holds 21psi to the redline