The increase in pressure only affects the shift quality and length. Driving with Performance Shift on will not hurt your transmission at all, if anything, shortening the time between shifts will actually help with the life of your transmission by reducing clutch slippage and heat build-up. The only real problem areas with our transmission are the differential and the input shaft. There are at least three options available for the differential (CYA, Intense, and Quaife) and Intense has a special input shaft that won't shear in half.

I've had Performance Shift turned on since I bought the car, and a Thrasher Shift Kit installed since about 15K miles set on medium. Under heavy acceleration, the shifts are firm and immediate with hardly any slippage. I've got 75K on the car now, and (knock on wood) no signs of and problems with the tranny.

One other word of advice, don't run synthetic fluid in the GTP 4T65E-HD tranny, it's been present in a number of transmissions that destructed, especially with the shift kit installed and set on maximum.