You may have a problem with the mains supply. In my last house, the water pressure was very poor, i had it looked into, and it turned out there was a problem in the plumbing between the street and my house. I mention it because a booster pump is supposed to bring the water pressure up to a point where your r/o is at its optimum, i.e, if you have poor pressure. So if your pump is only bringing it up to 40psi, to me that suggests either the water supply or the pump is the problem, but you are saying the pump is working but not getting enough water through.
As for you 450gpd system, i don't think i would bother with that as i cant imagine, from what i have read on here, it would be better that a 4040. I have a 400gpd system with twin built in booster pumps and it since i purchased it about 18 months ago, the best it has given me is approx 1 litre per minute, currently more like .75 litre per minute.