I think things can be explained a lot easier. I also don't believe the PU is even near Mercedes or Ferrari in terms of overall performance. Fact is that Mclaren performed best on 'twisty' circuits and sectors, not on fast circuits.
Their problem can be explained much easier and, for that, we need to look past the PR talk. If you take it word by word, they haven't really lied, but what the general public understands is completely different. Let me explain.
Firstly, let's take Arai's press releases before Spa. He said they already have an engine more powerful than the Renault and they want to get closer to Ferrari through this upgrade. Fact is that in terms of peak power, all engines are probably quite close. You have a turbo ICE which is flow limited, so all manufacturers will get very similar outputs and a
fixed 160Hp output from the MGU-K. That's it! In terms of top values, they are all probably easily within 50Hp, if not less.
No, the key here is sustainability. At Spa, the Mclarens lost all electrical energy after 1/2 a lap, having to spend the other half recharging the batteries. On the other hand, you have Mercedes and Ferrari which are able to use the MGU-H to harvest exhaust energy and use that to recharge the batteries and power the MGU-K on the long straights. With simplistic calculations (2MJ/lap MGU-K harvesting limit with 120KW delivery limit), you get only 17s of MGU-K full power delivery each lap. Rest of the lap is then spent being 160HP down. That's why Honda's pains are way bigger on power circuits like Spa and Monza where a lot of time is spent at full-throttle.
I have faith though. If their only major design flaw is now that they are not able to harvest enough from the MGU-H, that's easily rectifiable over winter, as Ferrari have shown last year. However, corporate mentality seems to be different in the two companies and this might be Honda's pitfall.
Whereas Ferrari realized the design flaw last year and by this time in 2014 they had already employed highly experienced hybrid engineers, Honda are taking the DIY route by trying to solve everything themselves, with no help from the outside. This is not necessarily bad if they are thinking of becoming a university or a training center for their engineers. Very bad idea otherwise. Having not designed a modern KERS system ever, having a year down on the other manufacturers, having a smaller budget, having no meaningful hybrid turbo experience and in a formula where upgrades are heavily regulated. Time will tell obviously, but I'm afraid their pride in not taking in outside help might destroy this partnership and Mclaren with it.