I went via the outfall when supporting on a clockwise round in 2003. Around the same time Nick Wharton told me he reckoned that descending from Dollywaggon Pike via the old fence line and contouring round above the tarn was quicker and he'd gone that way on his round. I've not gone any other way since as I really didn't like the direct line up Fairfield as it seemed to mainly consist of awkward ascent on scree. Maybe we got slightly the wrong line?
One other point to consider is the effect of lots of parties heading up that face, at the moment there is little sign of usage but the paths either side of Dunmail Raise show how quickly paths can come to exist on the ground. Going round via the tarn and the hause uses existing paths so should probably be preferred for environmental reasons. I know many of the coves on the east side of the Helvellyn ridge are ecologically sensitive but am not sure on that face of Fairfield.