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The performance improvement would come from running the engine at full revs, which could not be done safely previously.
Just imagine what 13.5K over 11.5K RPM in 8th gear can mean for top speed down the back straight in Miami!
Well, it will be better than the current 11.5K RPM. But team members have already stated the engine was not too far from running at full capacity anyway, so I expect any improvement to be minor until a real engine upgrade.
Considering that the maximum speed gap is generally around 20 km/h, the power gap shouldn't be that significant. In fact, considering the high speed losses that occur during clipping, and as I mentioned before, Alonso was saying at the start of the race that they have similar power to the others. So it's even possible to say there's no big difference compared to others except for regenaration defecit. (Here, when I say "no difference," I mean something like the power difference Ferrari compared to Merc, there can be a difference for Honda compared to Ferrari, for example). There's also the transmission ratios aspect. It's hard to find accurate information amidst all the empty talk circulating, but it seems to me that a dramatic power increase can't be expected. Even if the engine were running at full capacity, this car wouldn't be able to compete at the front. But we also need to keep in mind that McLaren, after a terrible start two years ago, suddenly became one of the fastest.