I'm of the cool, non-perspiring persuasion too. According to Polar Beat on my iPhone, working with an H7 Bluetooth HRM, over a particularly hard 8.71km day before yesterday I burned 227 calories. That appears to be more in line with what the OP reports; normally I have difficulty burning more than about 200-220 calories on this particular ride.
I'd be very keen to go hard enough to burn 522 calories in 8 miles.
Of course, I'm not vouching for the accuracy of any of these devices or calculations; like Andy, I just don't know enough about them.