What is the cause of the oil supply problems then........
Does the pump lack volume capacity?
Are the engine tolerances too tight for high rpm use?
Does the oil system lack overall pressure?
Poor oil drainage causing the sump to run low and cause oil pickup problems?
Pump volume not a big issue, even in the big block V-8 world they are learning that stock oil pumps usually deliver plenty of flow. Over sized high flow pumps actually can be worse than stock pumps. They use up power that could best be delivered to the crank shaft. They heat the oil more than the stock pump, as they are doing more work on the oil, They tend to agrivate oil drain back problems, as they do a much better job of emptying the oil pan and do nothing for oil drain back.
The engine tolerances are a bit tight for sustained high rpm operation. Race motor builders frequently open up bearing clearences to account for the increased heating and higher loadings seen in high rpm operation. Running too tight of a clearence is a one way ticket to an engine rebuild.
The stock oil pump pressure in the turbo engine is adequate up to about 8000 rpm, but due to the use of a cross drilled crank, oiling problems begin to crop up at that rpm due to problems in delivering adequate oil flow against the building centrifugal force which is trying to throw the oil out of the crank journals. When the counter pressure developed by the centrifugal forces equals the supplied oil pressure oil flow shuts off. Also the high centrifugal forces in a cross drilled crank journal tends to induce bubble formation in the oil at the center of the journal (where the oil feed hole to the connecting rod comes in) and this delivers aerated oil to the con rod.
Poor oil drain back to the sump is very high on the list of problems that need to be addressed. The Stock oil supply routing can pump large quantities of oil into the cylinder head and the oil drain back path is not designed to "assist" oil return to the pan. Using thick heavy weight oil to "protect the bearings under high load" actually makes this problem worse as it drains back slower than lighter oils and takes longer to get rid of air bubbles entrained in the oil.
Balancing is helpful but not the primary issue.
Detail work on the oil pump passages (and engine oil gallery passages) is how the high end racing engine builders keep adequate oil flow without jacking the oil pressure to the moon. Cosworth knows what their doing.
The secret to keeping an engine alive at sustained high rpm is not one magic component it is a lot little details and detail work on the oil system.
Part of the problem is that most folks are measuring their oil pressure at the wrong location. They are measuring it near the oil pump, oil filter, or the main oil pressure switch which is just past the pump.
In a racing engine you want to measure oil pressure at the farthest point of the oil delivery system. On our engines the oil supply to the heads/cams and the turbo are at the far end of the delivery system. It would take some experimentation to find the best possible tap point for measuring the oil pressure, but the rear of the cam galleries, or the turbo oil supply would be the two places I would look at first. By the time the oil OEM warning light comes on as system oil pressure coming out of the oil pump
drops below 2.1 psi (the set point for the pressure switch) you essentially have no oil pressure in the distant bearings. It is basically a "you just killed your engine light"
My suggestions for keeping a Subaru engine alive at high rpm.
Don't run a heavier oil than you need to.
Add an oil cooler (which adds to the oil system supply volume) and helps control oil temp.
Put a second oil pressure switch (and or gauge) and light mounted off the rear of the head oil galleries set at 15 psi or so.
Higher capacity oil pan, or an external reservoir such as an Accusump.
Run a crankshaft such as the STi drilling pattern or one of the after market designs that elminates the OEM cross drilled main design.
Do some detail work on the oil supply passages on the oil pump and the engine oil supply galleries.
Do some detail work on the cylinder heads and the oil drain back path to improve oil return to the pan. (paint the head surface in the valve cover with an oil shedding coating to speed oil return).
Use properly clearenced, high quality rods that are better able to keep the big end round under high enertial loadings at high rpm.
Get an engine builder that knows how to clearence the rods for a high rpm engine.
Properly baffel the oil pickup, and control its pickup to pan clearence.
Install a dry sump system if your serious about high rpm operation.
Understand how you are loading the engine and how that changes oil supply problems (ie a drag racer will have different needs than a road racer).