1) The yield rate for Dow chemical Filmtec membranes which are by far the most common is from 2 to 8% (it depend on the size) according to their own documentation. What this means is that you not only need to pump water through the membrane at pressure (800psi) but also at quantity. A high pressure washer does give the pressure but not the quantity as there is no need for them to waste more water than needed for their intended purpose - cleaning.
2) The materials used in Karcher type pumps are not made for salt water and will deteriorate quite quickly.
3) They are extreeeemely noisy to run on board a yacht.
I changed my pump to a General Pump piston pump (that is the make) which has a throughput of about 816 lit/hour. I have two SW30-2540 membranes with a stated recovery (yield) rate of 8% each, giving me a theoretical yield of 65 lit/hour each. My effective production is around 130 lit/hour which is pretty spot on what it should be.
The SW30-2540 has a maximum feed flow rate of 1360 lit/hour so I could have bought an even beefier pump and produced up to 216 lit/hour.
There is no magic to this regardless of what others might say, and the most important part of the system is the pump.