The head is fine to tap for the oil feed for the turbo, as the turbo doesn't require a huge amount of oil pressure to oil the bearings.
I am not taking oil from the main gallery, not after understanding how IMPORTANT it is for the rod bearings to get evewry drop of oil possible! I am using it fro the pressure reading only.
As for your oil pressure readings from the head, you will find that they will be lower than the true pressure in the bottom end. This is OK I guess, as you could see a potential drop in pressure which would alert you of trouble, but you are only getting "half of the story" so to speak.....
Also if you intend to use the same head tap for reading oil pressure and for feeding the turbo, you pressure readings will drop again and be even less accurate as you are effectively bleeding off pressure to the turbo now. Understand what I am saying?
