With those bigger flies I find it easier to cast if you slow everything down. When I first started out it seemed to take a year before the loop would open and the rod load. If you try to cast too fast you will lose all momentum and end up with a mess at your feet. Over-lining helps keep that energy through the cast, and the cast tends not to collapse as much.
Kschaefer3, Interesting you say that bc I believe you are right on with what's happening in the rod when you overline. Distance casting tournaments are won with slow action rods of great power. They store more energy and throw it more smoothly and efficiently.
People sometimes confuse power and action, evidenced by the seemingly contradictory idea that if you cut some of the tip off a rod it actually becomes slower in action, but feels so much stiffer. It has the same amount of power though. That tip is what's throwing the weight in faster rods, while a slower rod distributes the power over a longer length.
If you over-line (a properly rated rod) you have to reach deeper into the blank to support it, essentially slowing the action of the rod, and your casting stroke must slow down with it.
I agree with you that to throw a certain sized fly requires a certain amount of mass in the line to do it. One way is to overline (and adjust your stroke) or, go up in rod power -trade your 10 for an 11 or 12.