Get the water pressure gauge. A pressure gauge with a garden hose thread can go on the drain for the water heater to see if you are getting much pressure drop up to that point. Under $20, and maybe a lot under.
I know this thread is extremely long ;however, I posted the results last year.
I get just under 6 GPM with my shower head.
I had 72 PSI at the Water Heater while the shower was running at 6 GPM.
This measurement was recorded on Sunday night around 11:3opm Eastern time.
I had 66 PSI at the outside hose spigot (on the other side of the house) while the shower was running at 6 GPM.
This measurement was recorded 10 minutes later around 11:40pm Eastern time.
Visually, the water pressure looks MUCH MUCH MUCH stronger coming out of that hose spigot, than the water pressure in my shower head.
For example, I can barely place my hand underneath the hose spigot without my hand being pushed aside. I would love to have that kind of water pressure coming out of my shower head.
To fix my problem I suppose I want:
1.) Around 8-to-10 GPM coming out of my shower head.
2.) About 100-125 psi
***UPDATE**
A more recent test I performed in December 2017 gave me 72 PSI when I turned the basement shower on, which are the same results as last year. It was 78 PSI at the water heater before I turned on the basement shower or hose spigot.
Some suggest that 66 psi should drill a hole in the back of the shower wall; however, when I take the shower head off the shower arm I only see a tick solid rope of water arching halfway to the midpoint of the shower stall.
It is not a single-line straight blast of massive pressure drilling the back of the shower wall. I wish I had that!
The options of installing a brand new water heater or water pressure booster CVS pump/tank will cost me easily over $1500 with labor. Installing 1-inch copper pipe is also expensive , and I'm not sure it can even be done after the basement was finished.
I was naive thinking a simple shower head could solve this.