James, do you live in a flat? If this is yourself then you were being advised on how to produce around 400ltrs per hour (a lot of water in a short amount of time). To produce this quantity you would need a fairly high end RO (hence the quoted amount, not just a pump).
Also filling up in a flat your water pressure/flow could be pretty bad which would make anything seem better!! When you say a diaphragm pump, which make and model have you tested this with? A standard 12v van pole pump? If this is the case they only have a flow rate of 5ltrs per min which is well below the membranes minimum requirement (12ltrs per min), this would be a pretty amazing result if this is the case.