I finally had it with the Price Pfister 08 tract home cheapo valve. So I tore the drywall down behind the shower and now have full open access to valve. Wide open with no shower head it passes 4gpm. That's with a brand new cartridge. Without the cartridge, water flow is 30 gpm. So this valve somehow restricts the flow from 30 to 4 gpm.

I want to replace with a good quality valve that can do a true 8-12 gpm min. I want the shower head to limit to 2.2gpm, not the valve.

I was told to look at Grohe thermostatic valves and not pressure balanced ones. Can someone chime in and educate me please on why a thermostatic valve can do 18 gpm easy but pressure balanced are around 4-8 gpm max.

And finally, is there a gold standard shower valve that you pro's use?

Below is a picture of my attempt to increase flow by using both shower and tub outlets together to increase flow. Don't laugh at my skills, I'm a C-10 electrical contractor. Anyway, this didn't work.