Trek's 520 bottom bracket is actually 1 mm higher than their XO cyclocross bb, 27.7 vs. 27.6 cm. Besides, do you really think a few mm difference in bottom bracket height will impact the ride as much as how high your panniers sit and how you have them packed if too many of the heavy items are up top? IMHO, I think this is another one of those myths that gets regurgitated so often here it then becomes thought of as fact.