One thing we seem to often forget is that the 2014 to 2015 Ferrari big jump was not solely on engine, a lot of it was thanks to the chassis as well. I recall an interview with Allison in the beginning of the year that he stated that around 40% of the gains were in the chassis department. So in theory Honda would have to make a jump around 4/5 times the one Ferrari in engine alone. This of course if we believe the speeches the graphics, the estimates of improvement of Mercedes, etc.
But there is not a lot to do while the season is off, we can just speculate i guess.