According to Autosprint, Ferrari may have found 20 horsepower via a special paint that acts as an insulator delivering more heat to the turbine. Apparently they have Marussia to thank for the help in vetting the innovation.
This paint or Magic Paint, as Autosprint calls it, would be used at the upcoming high-power circuits such as Spa and Monza where top line speed is crucial.
According to the article, Ferrari tried to get a new engine modification approved for the Canadian Grand Prix which improved the heat of the air delivered to the turbine but the FIA did not approve the design. This new insulator is believed to be viewed as a reliability modification and therefore be approved by the FIA.
There is a fine line between outright engine design change and reliability and I can only guess as to how many requests and on what grounds these requests are made. If Ferrari can find 20 hp by using a special insulating paint, then that could help close their speed gap to Mercedes for the Belgian and Italian Grands Prix and the team could be thanking Marussia for their efforts in developing the solution.