Just some info. From the interview by a (some?) Japanese media, Arai give some insight on why Honda engine was slow. This is not a translation but basically my interpretation of what was written on the article. Basically the MGU-K only being used to lower fuel consumption. They can't really top up the energy storage, probably because the MGU-K wasn't being run at full capacity. Arai used the term plus minus zero, which probably meant the MGU-K is basically not filling the energy storage ,and again, only there to help with the fuel consumption. Also as the consequence of this, Button need to perform a lot of fuel saving himself (like coasting).
They also want to run with a (PU?) mapping system where if the temp goes high, the PU will run in lower performance. So basically they want to run full power and automatically (or manually?) throttle the PU if it got hot. But they found out that doing that could damage the PU, thus they are forced to run on a lower performance mapping (probably this detuned engine stuff). Basically they want to run at full power, but right now they just can't.