Will two seperate valves give better PSI?

CWTX wrote he installed two seperate Grohe valves one Tstat and one flow rather than a single pressure balance and claimed he has more flow out of the same head. I just installed a single pressure balance in my shower and had the option of the Tstat and passed, I hope I didn't make a mistake :-(

Sofa King

All depends on which valve you use:
Grohe 35.252.000 pressure balanced rough-in at 45psi will give you 6.8 gpm
Grohe 34.126.000 thermostatic valve with volume control at 45 psi 7.0 gpm.

So not much difference

The flow of showers is controlled by the shower head. The ones that meet current US standards usually have a little rubber device in them that deforms as the pressure increases to control the flow near the 2.5 GPM limit. If you leave that in, it doesn't matter much what valve you use as long as it delivers enough water.

In most cases the valve pressure/flow ratings should be read as:
The valve will deliver x.x gallons per minute with a pressure drop across the valve of yy psi. That result will be close to what you get delivering water to the bath faucet if it is a bath/shower combination valve. The amount delivered through a shower head that probably requires 20 psi to work decently will be somewhat less and is usually limited by the shower head as described above.