A 3/4" pipe can flow nearly twice as much water as a 1/2" pipe. If you calculate the area of the pipe ppening (pi * r^2), you'll see why. Trying to flow more than about 6gpm in a 1/2" line will lead to erosion. So, it depends on what your specs are on the devices. Because the radius is squared, a little change results in a much bigger area, lower restrictions, and higher flow capacity. Just bumping the 1/2" pipe back up to 3/4" to make the connection won't help (much) getting more water there. High restrictions will decrease the flow, and therefore the performance of the shower.
If the inlet piping to the WH is only 1/2", you may need to go way back to where it is 3/4".
What is the max gpm of the shower fixtures you bought and what else to you think may be flowing at the same time?