You do realise the reason for the oil getting up there in the first place is the issue? Hence the restrictors. Not just RB26's either, RB25 heads suffer the same thing etc.
Englarging the returns/drains is only one piece of the puzzle
Why do you change so often? If there is nothing actually wrong with the oil - changing all the time is just costing you money. I'd be surprised if you got it tested and after 500kms there was any degradation at all. Meaning you could use it for another 500km easily
Then changing every 1,000km (or even 2,0000) and perhaps use a better oil that might aid in alleviating and protect better as well - Just an idea
And as the others have said, have you actually done a leak down test?
Sounds like just comp tests you've done... Which won't neccesarily tell you the full story.