16-8-2 is commonly recommended for high temp high pressure 347. And often it is recommended as ferrite control filler despite the fact that ferrite generally stays pretty low in this stuff. It probably would have been an acceptable choice as it is more crack resistant than 347 fillers.
As for the 347 filler I wouldn't recommend cutting welds out with high ferrite, if that happens considering that the primary threat with high ferrite is sigma fomation. And with 347 this takes place between 1000 and 1700 degrees. 100's of hours required for it to form between 1000 and 1100.
I don't think you'll have a too little ferrite problem. So then you would need your customer to disposition the higher ferrite welds since you will have violated their spec.
As always ferrite is controlled by cooling rate. I would suspect that with a interpass of 350 that your cooling rate is sufficiently fast to ensure retained Delta ferrite.
Carl,
347 steel is a stabilized steel used for high temperature properties. ER347 filler metal is used for welding titanium and niobium stabilized steels of type 19Cr 10Ni Ti (Nb) or similar, that provides improved high temperature properties such as creep resistance, compared to low carbon non-stabilized materials. 347 is therefore primarily used where temperatures exceed 400C. The typical FN of the ER347 is around 7, so too high a FN is unlikely. Matching the stabilized 347 base metal with the stabilized 347 filler metal is acceptable, even recommended by many such as Avesta.