For RB450G the spec says to supply 10-28 V DC.
Assuming boards circuitry needs 5V for example, what is the impact of using 28 DC PSU instead of 10 V ?

Would 28 DC PSU cause more heat dissipation on the board ?
Or is the board circuitry 'smart' (e.g, like in switching PSU) and it uses only as much power as it needs regardless of input voltage ?

E,g, in ordre to provide 5 V 1 A (5 W) when using 20 V PSU, one would need to 'pulse' 20V 1A with 25% ration.

All Routerboards use switched DC-DC conversion. Thus not much heat no matter what the input voltage is.