I have not really noticed much drop in top speed with the 13/50 setup. I was curious about the different gearing , so I did a little calculation. With a 13/47 (stock) setup, the overall drive ratio in 6th gear is 9.003; with 13/50 it's 9.577, and 9.753 for 12/47. For a relative (if idealized) comparison this would mean that at 6500 rpm, assuming a tire diameter of 26 inches, you'd theoretically see 55.8 mph with the 13/47, 52.5 mph with 13/50 and 51.2 with 12/47.