If that shower head is the only thing (not a bunch of body sprays, etc. that could be on also), any 1/2" valve should be more than enough volume to provide full output of the shower. One thing you could do is remove the head, take a big bucket and measure how much water comes out in a minute (or less, then adjust to figure out the gpm). If it is more than the 2.5g, then the only way to make it better is to buy a different shower head. If it is less, then something's wrong...some valves have filter screens on their inputs. If these are partially plugged, it could restrict the overall volume available. Note, pressure, with no flow, is the same whether you have a soda straw sized pipe or a fire hose. Obviously, you can maintain the pressure to more things with the fire hose than you can with a soda straw. A 1/2" line can safely supply about 5-6gpm at normal house pressure (on each side - hot and cold). So, the total volume will change some based on the mix of hot and cold you have to get your desired temp. One thing about say a tankless system where you've got it adjusted so all hot is your desired shower temp...your total volume will be less since you can't add some volume from the cold side to temper it to your desired temperature.