For a D1 school, I believe it is between 5.5 - 6.0/7.0. Most players at the top echelon of D1 either go pro or are near pro level i.e Devarrman/Isner/Johnson etc. The minimum could be a 5.0, but at a competitive D1 I think it'd be more correct to be 5.5.
A competitive D2 is about 5.0 - 5.5, and a competitive D3 is probably 4.5 - 5.0.
The best of each division would only be average for the next level. There are always exceptions, but I think these are good estimates.