I hate to be the bearer of bad news but you will be outside the efficiency range of that blower at 15 psi. These blowers like to be used in the 6-7 psi boost range which is a 1.40-1.47 Pressure Ratio (PR). Take a look at the attached compressor map for a 5th Gen Eaton blower. Those islands that look like a mountain on a topographical map are efficiency islands. You always want to "stay on top of the mountain" as long as possible. If you fall off the mountain too far then adiabatic efficiency will suffer greatly, power will suffer, and safety too because you are pumping on heck of allot more heat into your engine which may heatsoak your AWIC and cause IAT's to skyrocket.
The pressure ratio (PR) if you ran 15psi boost would be 2.02 and your overall flow in M^3/hr would be about 1,200 if you revved to 5,500 RPM's. You would literally top out in the 56% efficiency zone and stay there until about 4,000 RPM's where you would fall off into 54% and then progress until you are entirely off of the map altogether about 500rpms before redline. The blower would also be spinning at about 14,500 RPM's too. At 20 psi you would never even start on the map being that you would be at a PR of 2.36. If you want to run that much boost then you would have to step up to a bigger blower. I would run the M90 to about a maximum of 11 psi on your 4.0 V6 engine. You can get away with higher boost pressures on smaller displacement engines but once you spin that blower so fast then you will be getting allot more heat and not much more gain. Porting the blower does help offset this but there is only so much a port can do.
At the 15-20psi boost level you would be better served by stepping up to an M112 blower from a 03-04 Cobra. The Cobra's do tend to run in the 2.02 pressure ratio range too once they have been pullied and done a few bolton mods but their AWIC systems are also much more efficient to fight to keep IAT's down as well. Stock they run 8psi boost which is a 1.54 PR.