Hi Marco,
the result is pretty nice and close to the maximum efficiency that you can have. Using a linear regulator with 31V input (if it doesn't burn before) has (5V/31V) 16% of efficiency because the input and output current are the same.
The frequency value is related to the power loss, bigger is the frequency lower is the efficiency. This because this type of devices has losses during the switching phase, rather during the ON or OFF phase the power that is lost is pretty low. On the other side, bigger frequencies gives you a lower ripple.
You may try to increase the efficiency with a lower frequency and a bigger output capacity, but I think that your result are good and also the output voltage is stable.
Thanks for sharing.
Regards,
Dario.