Your numbers are correct, although you'll probably need to use the 60 or 75 degree ampacity column, depending on the rating of your lugs/terminations. As I recall, the reason you can pump more current through (2) #500 MCMs than (1) #1000 MCM, even though their cross-sectional area is the same, is because of skin effect.
If we were using DC, the current density would be the same throughout the entire cross-section of the conductor. However, with AC, most of the current flows near the surface of the conductor. Because of this, the circumference of your conductors becomes more important than their cross-sectional area for determining ampacity.
For example, a 1000 MCM conductor has twice the cross-sectional area of a 500 MCM conductor, but its circumference is only about 1.4 times larger. And if you look at the ampacity table (310.16), you'll find that the ampacity of a 1000 MCM conductor is also about 1.4 times that of a 500 MCM conductor.
Similarly, a 1000 MCM conductor has a circumference that is about 1.8 times that of a 300 MCM conductor, and an ampacity that is about 1.9 times that of the 300 MCM. Notice that the ratios aren't quite as close as they were for the 1000 MCM vs. 500 MCM. As I understand it, this is because the skin effect becomes more pronounced the larger the conductors become.
This is actually the reason the NEC allows paralleling large conductors -- if we weren't allowed to parallel conductors, the wires would end up being ridiculously huge whenever a large ampacity was required. Instead of being able to use 3 sets of 400 MCMs or 4 sets of 250 MCMs, you would need to use 1 set of something like 3200 MCMs. That would be over 2.5 times the copper used for the 3 sets of 400 MCMs, and over 3 times the copper used for the 4 sets of 250 MCMs.
