You should always have (minimum) 1" pipe to tie into past the house valve so you will be able to run multiple things at the same time with no pressure losses. Your limited to 3/4" on electric hot water tanks so it behooves you to use 3/4" copper coming in and out with 1-2times larger pipe when Pex pipe is being used for it's high flow restrictions(friction loss)due to ID comparison between different types of pipe. Pressure can be lowered in the system when designed properly by the plumber(old school design)or your pressure needs adjustment (higher) to increase your flow rate and pressure for performance so you don't see pressure losses. Last thing on the pump installer would be elevation losses to increase your pressure settings. Elevation and small pipe being used causes high flow restrictions and pressure losses that can be seen when opening a tap and looking at a pressure gauge at the same time(a accurate pressure gauge by the pressure tank is not going read the same pressure at the tap due to friction losses through the pipe run) elbows, 45's, and going through different valves all have extra friction losses.

You need your water treatment guys on the same page as your plumbers when getting in with homebuilders for proper designing(not tring to save money by undersizing the pipe run).