Plumbing in the Home/Low Water pressure after installation of new hot water tank,expansion tank and regulator
Can unmatched expansion tank pressure cause a reduction in water pressure?
First, we need to define what you mean by "water pressure". Many people have some confusion about this. Water pressure is the static measurement of pressure within a water delivery system when nothing is running. On the other hand water flow, which most people equate to water pressure incorrectly, is the amount of water moving through the pipes and is measured in gallons per minute.
So, the question is are you experiencing a loss of pressure or loss of water flow. Although somewhat related, they are not the same as one is static and the other is dynamic.
Now for your question; a water heater expansion tank is not a pressure boosting system. It should not have any effect on the static water pressure within the system, on an ongoing basis or the flow rate for that matter. Most manufacturers like Watts and Therm-x-Trol ship their tanks with an internal pressure set at 40 psi by the factory. As long as the system is functioning correctly, this is the correct pressure. These tanks do fail occasionally and the internal diaphragm can be damaged such that the air pressure is lost and the tank becomes "flooded" meaning that there is no air cushion and the tank itself is full of water. Under these conditions, it does absolutely nothing but store 2-1/2 gallons of water.
These expansion tanks are only designed to compensate for a pressure increase due to expansion of water as it is heated. That's all they do. Without an expansion tank, system pressures can as much as double in some circumstances. This is potentially hazardous and is likely to cause leaks in your system. Temperature and pressure relief valves will begin to leak and other devices or appliances connected to the system are subject to excessive pressure. A diaphragm style expansion tank absorbs this temporary increase in pressure until the system normalizes with use.
If you are in fact experiencing an actual drop in pressure, by measurement, something else may be going on. Water pressure you receive at your house is typically controlled by the city water system and the amount of demand on the system at any one time. It varies throughout the day by as much as 20% or so. This is normal.
The most common factor for significant drop in pressure within a domestic water system is a pressure regulator failure or malfunction. If you are experiencing a pressure drop in your system, this is where I would begin my hunt for the problem.