I used to use a 60 degree feather on my wing paddle. Then I got interested in using Greenlan style paddles (not on a surf ski obviously) and these are not feathered. I found it difficult to adapt between a non-feathered and highly feathered paddle, so I switched to non-feathered paddling 100%. I don't race often but I do tend to go fast for exercise.
I paddle a racing kayak (Rapier 18), which is about as narrow at the catch (about 40cm at most) as the intermediate skis or some of the more slender beginner skis. I also use a wing paddle in wider kayaks that are about 45-48cm at the catch. The wing always has an advantage over a non-wing for going fast but it does diminish somewhat with increase in boat catch area width. Still, my GPS with heart rate monitor tells me that in any conditions on open water and in any of my kayaks the wing beats any other paddle I have for speed and power (of course, the other paddles have some other uses for which they are better suited, but a surf ski or fast racing kayak paddler would not care for them).
I do not think non-feathering decreases the efficiency of a wing one bit. The only thing it does is that non-feathered paddles do have more wind resistance in upwind conditions and that can be felt quite a bit and is meaningful. If one is racing and never plans to touch a non-feathered paddle, anything close to 60 should be good. Less than that and the effect diminshes somewhat.
With non-feathered paddle (if you have not done it) it takes some getting used to - there is no control hand and everything is symmetrical. There is some rotation of the shaft required relative to the wrist and it is required on both sides. This is a smaller amount compared to using a feathered paddle and there the rotation is on one hand only. Take your pick - same thing IMO.
To sum-up - the advantage of feathering to me comes from the air resistance decrease upwind. I don't believe a non-eathered paddle loses otherwise against a feathered one.
If someone has a good theory to the contrary, let me know why - I'm not that much of an expert myself -
. As to wing vs. non-wing - there is a huge difference regardless of feather angle. To me, the feather angle has nothing to do with how you use the paddle in the water - feathering only affects what you do with the paddle out of the water, so the in-water efficiency is not impacted in any way (at least it should not, if you plant it well and pull it properly).