What factors need to be considered to get the most powerful shower possible?
Official term is "water velocity" but for domestic plumbing this isn't really a thing you need to concern yourself with stick to pressure and flow rates. To answer some of your questions all in one: 
The flow rate can vary between showers due to the plumbing that supply's them also the pressure can vary for the same reason, they can also have water restrictors  that slow the flow rate down.

In Australia this is due to regs stipulating maximum flow rates for water conservation reasons, ( I will always remove these as I hate a weak shower).

As for the flow rate you can use a flow cup to check the flow rate on a unrestricted tap and a pressure gauge to check pressure available.



You can attach a booster pump to your water supply but if this is legal in your location I do not know, the use of a booster pump will also depend on the supply pipe you have to connect to and what the pressure is already if it is near the output of the pump there is no point installing the pump.