To the OP:
Don't confuse what is ethical with what is 'sporting'. IMO, the emphasis on wing shooting is a 'sporting' concept that has nothing whatsoever to do with ethics. I don't know for sure, but I think the 'sporting' concept comes from old world hunting of planted birds in an open field. The only similarity to this with hunting wild partridge in the Canadian bush is that you're shooting at birds with a shotgun. The only ethical questions you should ask yourself are 1) Can I kill that bird quickly and humanely?, and 2) Can I do so without damaging the meat? Clearly, head shots on more or less stationary birds on the ground or in a tree are potentially more ethical when both of these factors are taken into consideration, assuming you're not using too much gun.
Personally, I only pull the trigger on a wing shot if I'm certain I can drop the bird, maybe 2 or 3 times per season. Most of the wing shots that I try are successful. When I miss one, I will scour the area for feathers to make sure I haven't wounded a bird. I hunt mainly with a .410 to minimize damage to the meat and pass up a lot of shots on flushed birds where the range or the cover might exceed the limitations of my gun. When this happens, I will stalk the bird and often get a 2nd, 3rd, even 4th chance for a shooting opportunity. Maybe I'm overly conservative on some of the shots that I pass up, but I'm pretty good at stalking and finding them again, and I do pretty darn good.
I guess to sum up, I would suggest this: If you want to limit yourself to wing shooting, recognize that it is not an ethical decision, but please bear in mind the true ethical considerations I outlined above.