I have just ordered one of the following to evaluate:
http://www.ebay.com/itm/LM2596-DC-Co...item2a18fae6eb
4 to 40v input, adjustable 1.5 to 35v out.
Looking at the datasheet for the LM2596, it should run at about 85% efficiency with 12v input 8v output.
Being a switching regulator I will test it to see how clean the output is and the stability of the regulation, also how much RFI it generates before putting it into service.