Again, my own anecdotal experience really does influence me more than a study. You know that old saying "should I believe you or my lying eyes?"...
I have a dog friend with both "pure" and BC Dals - no difference, well, actually, I think the BC Dals she has have better markings, technically, than the "pure" ones...
That's one thing about having a limited genepool that is worrisome. Nutrition aside, our basic building blocks are the genetics.
And as far as food goes, really - I use premium foods. I know lots of folks who have dogs with food allergies, intolerances, etc...
Not that I recommend brands like Ol' Roy, but I don't like to see food allergies/intolerances as "acceptable." A dog really should be able to eat anything. "Soy" is not a four-letter word! I don't like the idea of breeding dogs with such issues - immune-related, no doubt.
There are some recipes I wouldn't feed my dogs, but I also would be very wary of keeping dogs that need specialized diets in the gene pool. No better than a hothouse flower!
Certainly a wild dog/wolf with a food allergy would be weeded out of the gene pool...