FWIW, industry says that 5-6fps flow velocity is about the fastest you can run water through a 1/2" supply line without having long-term problems. That amounts to under 4gpm for a single 1/2" supply line. Since you have both hot and cold, a max might be around 8gpm. Short sections of narrower restrictions cause the water to speed up (venture effect) to compensate, and the flow rate doesn't change much unless there are a lot of them, or they're longer. How a 1/2" valve can claim over 10gpm is not very realistic..I'd like to see their test results. You might be able to do that if your pressure was excessive, but the flow velocity would be beyond maximum for industry standards...it's not like they can reinvent the rules here.
Most reliable, realistic 1/2" shower/tub valves are rated 6-7gpm. And, you'll only achieve that when your water is using almost a 50-50 mix of hot and cold which is unrealistic, especially in the winter. When your mix ends up more like 80-20 hot/cold in the winter, the max flow rate goes down.
A 1/2" valve is typically only good for two heads max, and depending on your supply lines (pex or cpvc would be pushing even that) and pressure, may not provide great results even with that.
As is discussed a lot here, if you have the vertical height to install one, a waste water heat recovery system could extend your shower with the same WH at a significant energy savings verses a bigger WH, and lengthen the available shower time. A little searching will show you some of this info.