Guys,
I have had extensive experience with Porsche's and their dry sump systems so I will fill you in on what I know.
As many may realize Porsche's are a boxer engine design. With that in mind, there is really no "oil pan" per say. Jimbo's point about the crankshaft is exactly correct, in that when the crank/rods are moving through a "bath" of oil it robs HP.
In the boxer engine, without an oil pan, this type of system has hugh performance advantages, probably more then an engine with a oil pan, like the NSX. However there is still some gains to be had.
The "Dry Sump" system uses two oil pumps. One is a "scavenge pump" and one is a "pressure pump". The scavange pump pulls oil out of the crankcase and sends it to a holding tank. The purpose of the holding tanks is to let the oil settle and remove all air bubbles. The output of the tank is then a perfectly solid flow of oil to the pressure pump that pumps the oil to the bearing, heads etc.
Yes, this system holds 13 qt's! Makes for a pretty $$ oil change sometimes. But... since, in the case of a 911 engine, it is air cooled, the oil plays more of a role in cooling then a water cooled engine.
Doing this to an NSX motor is definately doable, but it will take some engineering, packaging, probably a better oil cooler, add a tank, etc. Last time I looked there is not a lot a free space in an NSX engine bay.
I had a discussion with Andrew, who is the head mechanic for RealTime racing, about this topic at the Memorial Day NSXCA NE event. The reason they did not do it, is because they would incur a wieght penalty for the increased horepower. Since on the street we have no "rules" there would be some tangible gains although I cannot begin the quantify them.
This system does also cure high G oil starvation. Oh, this oil pump does not have to be external. Porsche engines have a tandem pump in the lower section of the crankcase.
The challange with that is if you have an oil pressure problem, the whole engine must be torn down to get to the pumps. The NSX engine "wet sumped" is easier to get to the oil pump. As many who have overrev'd already know.
HTH,
LarryB