The newer design of switchers achieve much better performance than the LM2576. The duty cycle of a buck switcher will be low (it's Vout/Vin), so I would look for a buck regulator that features synchronous rectification. TI do a bunch of them, and they promise >90% efficiency. I would also choose one with a switching frequency of at least 250 kHz, probably even at least 500kHz as this will improve transient response.