In on/off control since the peak current is fixed (and will be reached every cycle regardless of anything else) the chooice of device will dictate the value of peak current. Therefore for your application if you want to operate more continuous you will need to choose a smaller device.
This is why for on off control based designs KP is not a user input, its a calculation.
To go more continuous choose a smaller device. However a smaller device has a larger RDSon so whatever benefit you may have in decreased RMS currents you will offset at least to some degree with increased RDSon. Net net you may be in the same efficiency range.