Ferrari is reporting that they have picked up 40hp just in fuel+combustion chamber optimization. Supposedly the Mcl's last year running the Merc engines were down almost that same amount of HP to the factory team because of their obligation to use Mobil fuel, which has not been optimized for the engine.
My take: Honda isn't working with Mobil closely enough. They also got the MGU-K sizing wrong, and apparently its ability to dissipate heat wrong.
Honda has already changed their 'combustion concept' once this year according to Arai and it didn't seem to show much in the way of increased performance. I am convinced that the name of the game with the 1.6's is to get as efficient of fuel burn as possible so you can run max boost without detonation, and generate a lot of hot gasses to the turbine. The only way to do this as well as Merc and Ferrari is to have your own fuel and develop it in conjunction with the combustion concept.