I also suspect that a high GPS output rate might not be needed.
A mountain bike often goes quite slowly at times. At a 10hz sampling rate, and given the normal variations in position errors, the GPS might think its not moved much and thus the indicated apparent speed could drop to very low levels for short periods.
So you will probaly end up having to smooth out the speed part of the data as it may change quite a bit, so why not let the GPS do the smoothing by cutting the refresh rate.