from what u are saying it can be that u will get more oil to the head by less oil the crank(i didnt look at the oil pump passage ).
dont forget that the pressure drop is after the banjo bolt and there is some pressure before.
I am saying that because of the cold, viscous oil, and the small oil line + banjo bolt diameter, you get a pretty big pressure drop when cold. This means pressure drop at the end of the tupe but pressure buildup at the beginning.
If the crank diameter is bigger, a lot more oil will flow there until warming up because of less resistance... altough I have to look into this by measuring passageways to the crank. I'll have a look this afternoon.
When you increase the diameter of oil line to the head, but keep pretty much the same size banjo bolts the pressure drop when cold becomes much less because of less wall drag/friction. So that might have good effect without removing the restrictions of the banjo bolts.
I'll look into this more when I have more time.