I have carried 30 pounds in two rear panniers while climbing extremely steep hills -- up to 18% grades. Sometimes it feels like the front wheel will come off the ground, but it never has. I compensate for the (perceived) imbalance by positioning myself low and forward while ascending monster hills.

I try to put heavy stuff at the bottom, and lighter stuff at the top of the panniers. Raising the centre of gravity makes a bicycle feel skittish to me.