Can you provide a link to these filter manufacturer specs you refer to? I think you may have misunderstood what the ratings mean.
In my experience, normally filters are sized by flow rate and dirt load, not by pressure. Every filter material has an optimum flow rate per unit of surface area. This flow rate gets adjusted based on the properties of the fluid being pumped and the level of contamination in the fluid. When the fluid is relatively clean the flow rates per unit surface area tend to be increased. Dirty fluids need much lower flow rates per unit area to give a reasonable run time before the filter needs to be changed.
The pressure drop across the filter element is the difference between the filter inlet pressure minus the filter outlet pressure and is commonly referred to as the differential pressure. The differential pressure is important to consider when selecting the pump for a filter system or when retrofitting a filter to an existing pumping system. The maximum expected differential pressure must be added to all the other pressure losses in the system (from friction in the piping, valves, etc) and to the final pressure required at the point of use. This number is then used to determine the total pressure the pump must be able to produce at the desired flow rate. If there is an existing pump which cannot produce enough pressure, then a larger filter with a lower differential pressure at the desired flow rate must be chosen.
Other than the initial pump sizing, the differential pressure across the element is typically used for determining when the filter is dirty. I have seen manufacturers set limits on how high the differential pressure should go before the filtration quality will degrade or before the filter element may fail. I am confused how you can use the filter manufacturer’s differential pressure at a specific flow rate to determine the 16 gpm to 5 to 6 gpm flow reduction.
Using your example, if the pump is capable of 16 gpm at 5 psi and the filter is rated for 16 gpm with a differential pressure (pressure drop) of 3 psi, then the net result out of the filter will be 16 gpm at 2 psi. Note the pressure on the downstream side of the filter is reduced, not the flow rate. If you change the pump to one that does 16 gpm at 15 psi then the result out of the filter will be 16 gpm at 12 psi.
Now if you try to increase the flow to 20 gpm the differential pressure will increase and the filtration quality may degrade compared to 16 gpm. If the filter differential pressure at 20 gpm is 5. 5 psi, then a 5 psi pump cannot produce enough pressure to force 20 gpm through the filter and the flow will be reduced. Also keep in mind that as the differential pressure increases the rest of the fuel system and the properties of the pump will affect the flow rate, but those are not filter issues.
You are on the right track that usually the lower the flow through a given filter the better the filtration, but only to a point. Keep in mind that a dirty filter usually gives better filtration due to the build up of particles which partially block the holes or passages or “pores” in the filter media. This has the effect of making the pores in the filter media smaller, so that ever finer particles are captured. This blocking of the pores reduces the open area that the liquid has to flow through, like reducing down from a larger pipe to one that is getting smaller and smaller. The pressure needed to push the same volume of fuel through this ever decreasing open area will increase. This will be seen as an increase in the differential pressure.
Remember, pressure is a measure of the resistance to flow. Differential pressure, which is the pressure drop across the filter element, is the result of the flow being restricted while going through the filter element. It is dependent on flow only, not the supply pressure. The more flow, the more pressure that is needed to force it through the filter element and the higher the differential pressure. If the flow rate remains constant, but the filter’s open area is reduced due to dirt blocking the pores, then more pressure will be needed to force the flow through the fewer and smaller openings in the filter element and the differential pressure will go up.
What happens to the flow rate as the differential pressure goes up is dependent on the pump type, what pressure relief device the system has and if the filter has a pressure relief bypass built in. The filter media will respond to the increased pressure in some way as well.
In a nutshell, with a typical system, as the differential pressure increases the flow rate through the filter decreases. A differential pressure higher than the filter manufacturer’s specs is a sign the filter is dirty or, if the filter is clean, that it is undersized for the flow rate. I hope that helps to clear things up.
Attention: TDR Forum Junkies 
. Nick
