I can't give you accurate efficiency values... an wild estimate for 20V/20A out might be:
A) major losses in mosfet, if peak current might be 4A then peak Pdiss is 4A^2 * 800m-ohms = 13W. Peak output diode loss might be 40W
B) peak PNP loss might be 0.25A * 50V = 12.5W for driver and 20A * 2V = 40W (if Vce-sat is 2V) for the transistor itself. Output diode same as for A. There's also loss in the input transformer + magnetising current.
The components will dissipate some fraction of the peak power due to each having <100% duty cycle, and because the current waveforms are not square. If 'B' is simpler for you, you can boost its efficiency by using a mosfet for the switching transistor and a synchronous rectifier (i.e. use a mosfet to replace the output diode). You can get controller ICs that have appropriate synchronous driver circuitry integrated.