From the makers website: "Temperature and pressure ratings on PEX pipe include 160 psi @ 73.4°F and 100 psi @ 180°F; some pipes are also rated at 80 psi @ 200°F for continuous2 operation, with minimum burst pressure ratings as high as 475 psi @ 73.4°F. PEX pipes resist corrosion, erosion and abrasion encountered during installation."
That being said, I would not go much over 60 PSI because of other things in the system such as fittings, valves, clamps, etc, where the pressure rating is unknown.
Changing the shower head may also increase flow, and make the wife happy.