Former DIII National Champ. Glad I went DIII, got to have a balanced life with school, running, and other pursuits. Got a more relaxed environment in which to train (didn't "have to produce" for a scholarship, not that I was good enough to get one in high school). I don't know if I would have been faster in DI--maybe between teammates and faster races to get in to my PRs would come down, but I've seen enough blue chip talent get run ragged at that level to think it is guaranteed. I think the less laser-focused approach to running may have even aided my progression. Practice was 2.5 hours a day, not 4 with all the extra stuff and missing two days of a class a week to travel to some indoor meet 6 hours away. The only thing I did miss out on was faster races, because there weren't a lot that would have been feasible to get to and that were open to letting D3 athletes into their fast heats.
So, yeah, I guess I enjoyed my time, and my school set me up for professional success, so I really have no regrets.