There's been a couple discussions on this over the years on here. Here 30" corn is dominant. There are a few guys that plant 20" corn in my immediate area. There is really no yield differential between the row spacing, yet. My cousin runs 20" and some years will top my and some years I top him usually about the same, also 7 miles apart weather is a big a influence as any yet.
I think as hybrids are bred into higher population race horse breeds 20" rows will become more popular. In 30" rows our populations are maxed out around 38,000 I'd venture to guess, 20" rows can handle 40,000+ maybe push to 50,000. The big players in the seed industry are not yet breeding or really even attempting to breed corn for 20" high population rows, other than Stine Seed Co. who is trying to push the envelop a bit further. They have as good a shot as any being the creators of Liberty and Enlist beans they're not new to breaking new ground.
All that said from what I've heard is the further north you go 20" row spacing starts to out perform 30" slightly better. I understand you guys don't push the populations like we do and 20" allows you to do a bit more. I think Swman did a side by side sometime, hopefully he didn't get lost in the shuffle here and can pitch in eventually.