It's not clear to me why it would be especially wasteful to use the Arduino to regulate the voltage (I'm a biologist and can barely work my TV remote, so bear with me :)). So it would be more efficient to find a different DC-DC converter and use that--like a 12V-5V or 6V-5V? What is it about the Arduino volatage regulation that makes it inefficient compared to other methods?
Thanks
Jonny