The pressure drop is minimal...but it all depends on the oil cooler setup. Factors:
-Length of plumbing: the longer, the lower the pressure (pressure = force/area...force remains same, area increases)
-Size of cooler (same as above)
-Routing of plumbing and use of fittings: small radius turns or bends tend to lower pressure
Unfortunately, draining 100% oil from the lines is impossible. The system is setup such that ~80% or so of the lines are higher than where they come out of the block, so that oil just drains back into the pan once the engine is shut off. The remaining 20% before the remote filter is stuck in there... as is the oil remaining in the thermostat and cooler. I have thought of a drain plug or valve somewhere in the low part of the system, but I HATE re-plumbing and making new fittings, etc. The SS braid and Aeroquip fittings are nice and leak-free and easy to work with.....but when it comes to making them, and cutting the SS lines...painful to see the least. Either way...it may or may not happen eventually. HOWEVER- if you put a valve in there on the original design, it would be 1000 times easier than doing it after the fact. I'm really not too concerned with it....there's no way to ever get 100% out anyway and I change religiously (usually...). If you look at a lot of the OEM oil cooler loops (Porsche, VW, trucks etc) many of them do not allow for a complete oil drain during changes.
Also of note (while I don't think this matters too much for you)- if you ever need a quick easy access feed for oil...like say to a turbo- having the plumbing up front make this uber-easy.
Both my temp and pressure senders are located in the remote filter adapter (thats the whole reason I purchased the one with 4 ports) as seen here:
When I put the morosso pan back on the motor, I plan to move the temp sensor there for more accurate engine oil temp readings.
I noticed a ~8-10psi drop on max pressure yesterday...had been sitting for 3 days during the move. I'm thinking oil change is needed BADLY- been neglected for wedding/buying house/packing/moving etc. Filter must be toast.