Since your Vin max is only 300V and the power only 200W a straight buck converter (or resonant buck converter) with a mosfet in parallel with the freewheel diode (for synchronous buck operation) will give the highest efficiency as only one semi-conductor is in series with the current at any time, you will need some good caps on the input to supply the 15A pulses, but the mosfets need only be 350-400V, 40A rated (or bigger), a resonant buck will give lower switching losses than a straight buck and can be synch switched also. Interleaving buck converters will alleviate the stress on the input caps, you could have 2,3 or 4 stages, running at 50kHz say (to keep switching losses to a minimum), IC's exist for implementing all of the above - high side gate drive needed to run the buck FET.
Regards, Orson Cart.