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300lbs here and looking at that profile, 300 feet in 1/2 mile at an 11% grade is a significant descent but at an average speed of only 18.7 over that segment you seemed to handle breaking better than you may think.
I would mirror some of the others in saying pick your top comfort speed, once you get there brake hard to scrub speed, rinse and repeat. As for me and my bike, at my size I can almost initiate a front wheel skid (similar to on our tandem) without the rear wheel lifting and find the front does a whole lot more to stop than the rear. I run Ultegra 6700 brakes on the single and Avid BB7 discs on the tandem. With the tandem I have (only once though before I was familiar with the bikes/brakes) gotten the front disc heated to where it was glowing and chewed thru the pads in seconds causing fear and borderline panic with the stoker. I have gotten much better descending since that event.