Update...
I’ve contacted PF re;booster pump. Who better to ask as the RO is from them. I believe they do an aquatec one for the 200/300/450 gpd.
Also did a tap TDS check and comes out at 336 TDS so not quite 400 but I’m assuming that doesn’t change anything in terms of not needing to change the membranes?
Not sure if PF do membranes (have emailed them also) but will defo give Doug a call as per previous post 👍