ive got the pure freedom 4040, not long after being its found out just how crap my water pressure is, its about 8psi, i then bought a booster pump and fitted it between the pre filters and the membrane, i suffered with air locks a lot and also my booster would only go to 30psi max, ive now changed it round so the booster is first and the pressure is a million times better. I just need to re-secure all my fittings as ive got some leaks and then i think I can get my pressure up to 60 ish psi which is a massive improvement .
so in answer to where the booster goes, I think a lot has to do with your initial water pressure