Are newer gas heaters more efficient?
They can be. As technology advances gas heaters become more economic. The Omega Altise Briganger, Rinnai Avenger 25 and Rinnai Granada 252 mentioned on our list all boast energy ratings of 5.8 stars or higher.
That said, there is always going to be a variety of heaters on the market and you'll find some that are more efficient than others.