What type of valve optimizes water pressure in new low-flow showerheads?
Even a "standard" showerhead outputs only 2.5gpm at household water pressure.  Hence, given their specifications, neither of the valves you list are going to be a bottleneck in the waterflow to even two standard showerheads simultaneously.

Water volume and pressure drop through your existing pipes depend on a number of factors that can tricky to calculate, but unless you're willing to replumb back to larger-diameter pipes then those questions are irrelevant: 1/2" pipe is unlikely to be a bottleneck, but you can't be sure without pressure-testing a mix across the final temperatures you want to supply.