It's possible, but unlikely.
Chips are binned based on the highest stable clock at stock voltages, and oftentimes, higher binned chips are set at the lower speed when they have too many to sell at the higher price-point. Unfortunately, as far as I am aware, places like the Silicon lottery don't test for efficiency either, so you're not likely to be able to find a more efficient chip. Even if you could, though, it won't be a very large difference anyways, so I wouldn't worry about it.
You are better off focusing your search on getting a more efficient GPU, and the ITX versions sometimes ARE binned for lower power use.