Before you spend thousands of dollars paying a firm to fix your water pressure problems by re-piping, consider this:
Many years ago, I had low water pressure from my city water system. My step father, who was an old plumber, fixed the problem by replacing exactly ONE pipe: the first pipe into the house. When we took it off, you could barely see daylight through it. All the rest of my pipes were fine.
He explained that water pressure problems usually stem from pipes filling with hard water deposits, and that the deposits usually build up in order, starting with the first pipe. In fact, in his experience, it was almost always JUST the first pipe that was clogged.
So the fix is to take off one pipe at a time, replace it if it is clogged, and stop when you find a clear pipe.
Don't spend thousands of dollars on more work than you need!