Hill is offering about 5% extra performance over the Gazelle which was developed in the 1960s.
He's saying 5% in public because it's easily achievable. He doesn't want to be quoted on what he personally thinks will be achieved. That said, I was the first industry journalist to interview him back in November 2020 (see
Exclusive interview – Jason Hill, founder of Hill Helicopters) and in that he refers to the Bell 533 as a great example of innovation being seemingly ignored by an OEM
In March 1963, the Bell 533 achieved a straight-and-level speed of 278kmh, 25% better than the standard UH-1B, yet the benefits were never rolled out – fared-in rotor cuffs and mast, skid legs that weren’t tubular and so on. Not only were they never applied to the Huey, but they have never been meaningfully applied in small helicopter design.
No, I'm not saying he will achieve 25% better speed than any particular helicopter, but this just gives an insight to how he thinks.