66 or 72 PSI should peel your skin off in the shower. You will have the same pressure on the outside spigot as does the shower. The only difference is the pipes to the shower are causing a restriction. I would bet if you put a 3 GPM shower head on it will have lots of pressure. But when you try to use 6+ GPM the friction loss in the pipes to the shower are reducing the pressure at the shower head. So like you said you will either need to increase the pipe size to the shower or boost the pressure. You won't have as much incoming pressure when the pump starts sucking on the line. So if you have 66 PSI now, I would figure about 40 coming into the pump. So you need a booster pump that can produce 10-12 GPM and boost the pressure from 40 PSI to 120 PSI. You could also just put the booster on the line to the shower and nothing else. That way the rest of the house would not have 120 PSI on it, and the pump would not have to be larger to keep up with the shower plus the other uses in the house at the same time.