LTC7813 buck-boost IC max output power
The LTC7813 has external switches so the amount of power is determined by the inductor, mosfets, the sense resistor and parasitic values of these components. Use LT power cad to determine how much power the LTC7813 can handle with external components. You may be able to parallel the switching mosfets to get lower switching resistances and lower power dissipation per part.

If you search for 24V output with 15A the LTC7813 comes up on the list in LTpowercad