A very short section of 1/2" pipe would not restrict the flow too much compared to all 3/4", but a 10' section means you have essentially a 1/2" line if I understand it properly. Higher flow rates in a 1/2" pipe can cause erosion and noise issues, so getting more out of a 1/2" valve is not likely. One way to maximize it is to make the hot hotter...this means that you'll use more cold to get it the proper temperature for your shower. On a more common situation, with the WH set to a typical (safe) 120-degree max, you are using mostly hot in the winter, and only a little cold to temper it down to where you want it. But, if the hot is hotter, you'd use more cold. The flow from both the hot and the cold together would be higher. The internal passages aren't all that large, so it may not end up making all that much difference. Much better to go with 3/4" for what you are trying to do.