i run at approx 16 - 17 lpm @ around 210 bar with my 21 inch mosmatic and it cuts through moss great, you just need to run at a snails pace, an 18 inch whirlaway works even better but covers a smaller area, althought there will always be places where it didnt quite cut it enough, hence you go over with a turbo... although, the best thing that I find to do on heavily mossy / weedy block paving is just hit it with a turbo from the start, much quicker and effective, save your surface cleaner for flags,PIC, less mossier Block paving etc.
I'm also not sure if the 170 bar may cut enough, flow rate is great for cleaning but you still need the psi behind it to give it the cutting power, other wise everyone would be running 20 bar machines that do 60 lpm
