Upper 400s are *good* in D. There's just not enough strong regular competition to get much lower than the upper 400s and I strongly suspect that as you approach the 400s in D, you're probably right on the cusp of getting upgraded anyway (as I think is the case for you, Steve and myself).
Points are not category specific and if you're just starting in a higher category they can be highly misleading because the points reflect your best race performances over a ~3 month period -- so it's going to be including results from racing in the lower category. After a few months in the upper category, the points should be a more accurate reflection of your positioning within the higher category.
^ about sums it up. The Top D has a rank of 431, so you'd never get above that racing in D.