Tim...good question.

Personally, locally, I think the population control & depth control are the main issues vs spacing. The lack of uniformity in seed seems to cause the population control in non-singulating meter drills.....here.

There is an article in the latest edition of IL AgriNews w/quotes from the U of IL saying that population had little or no effect on yields. They were talking pops down to 100K, I believe. I assume that is emerged population.