What I learned from water pressure this weekend when I removed my old water heater:
I turned off the WH ball valve.
When I started to drain it, opening all the faucets in the house had little effect on the water quantity coming out of the hose. When I turned the TPRV to the middle position it allowed quick draining (1 hour instead of 4).
Since it was draining, and since the WH valve was off, I needed to wash my soiled hands. I turned on the house main to wash my hands.
I had a drip in one of the bath faucets in one of the bathrooms, which I heard as I was watching TV while the WH drained. As I'm watching the drip the toilet inlet water hose splashed some water on the floor. What the heck???
I go over to the WH and notice that water is going back into the tank through the hot water line. What the heck?
It seems that the mixing valves on my bathtub/showers are allowing cold water to pass through to the hot water line. Because the faucet isn't turned on the hole between the cold and hot water mixing valve is small - so the water pressure went up.
I turned off the house main, removed the old ball valve on the WH cold water inlet, and installed ball cocks on both the cold and hot WH inlet pipes at the wall.
When I turned on house main I no longer had the high water pressure.
So, what am I getting at? You have high water pressure. It may be that you need to replace all the mixing valves in your house (kitchen sink where you have a single handle to control the cold/hot water coming out of it, or the mixing valves in your bathrooms, either at the vanities or the bathtub/shower, if you have them at all. You may even have one, although very unlikely, at the clothes washer. It may be that even though the handle is turned off that the mixing valve is allowing cold water to pass through back into the hot water line. That would create high back pressure at the WH, and once there is any type of small leak at the WH the pressure will actually increase.
Once, a radio announcer asked why when first turning on a shower the water in AZ feels hot. It's because of the mixing valves. It was 100ºF yesterday in Phoenix. When I went to take a shower at 9PM all I had available was cold water (because I haven't finished installing the WH), and let me tell you - the water was cold. I know that once I turn on the WH the water will go back to "hot" the second I turn on my shower. And that's why it's better if you don't have mixing valves in the home, in the kitchen or in the bathrooms. If you have any leaks on any of the mixing valves anywhere in the house - that's where to start replacing them.