Pressure is directly related to resistance to flow. If you have a more restricted flow the pressure will go up. Thus no filter will have less resisdence to flow than even a hose and thus less pressure.
That said the oil pressure purpose is not to just move oil from tank to filter and back, the pressure is also insure that the proper amount is flowing to the bearing, rockers, push rods as well as have sufficent flow to allow for proper cooling of the pistons and cylinders.
Our engines do not require a great deal of pressue, but they do require pressure nonetheless. As the RPM come up it requires even more, so see like 10 PIS at idle in not that unusal, but if you were to see 10 PSI at 3000 RPM, there is a problem.
As far as scavenging oil systems, be it Rouge, CORS, or open flow and a drip pan, the reason is to just remove as much dirty oil before topping it off with new oil. Does it make that much difference, not sure but it will not hurt.
So for the price of half a quart of oil, a few rags to wipe down the leakage and maybe $20-50 for a easy tool, minor IMHO compared to the possible benifit of getting the most out of engine that list for close to $9,000 list.
