Finn, I would suspect that your pressure is low because your turbo is theiving oil pressure. One solution might be to restrict the oil return orafice after the turbo, slowwing down the pressure loss.
I have been told that L series oil pumps are the same diameter. If this is the case, then you could get another A series oil pump housing and plart off a peice of it in a lathe, to make up for the L series oil pump having a thicker gear set. then using the Lseries pump gears, with the modifeid Asreis housings, you sould have a higher volume pump.

If anyone can verify for sure that the pump gears are the same from A, to L, then speak up.