The only problem with the sunflowers is residue on the surface. The C:N ratio on sunflowers is fairly wide. You'll need to cultivate in the residue to help with decomposition. This will likely temporarily draw down soil N levels as the N becomes immobilized. Plus, you have the added problem of sunflower stalks on tractor tires. Still, it's a really neat idea and I'm glad to see more of this taking place. I actually planted some sunflowers in the middle of one of my E-fenced bean food plot this year. So far, I'm really happy with it.
In my line of work, I actually met a farmer near Dubuque, IA who was intercropping 60" corn with a similar combo of cover crops (minus the buckwheat). The previous year he yielded 300 bu/ac with organic (!) corn. Absolutely the future of ag is going to be using a system like this.