Hi Andy
The secret is out - Fujifilm has long been making some very fine lenses! Last month, I had the opportunity to use the 150-600 on the new X-H2S when I went on a photowalk with Fujifilm at Cameraworld Live which was held at Brands Hatch. As you say, it was a remarkably light combination given the lens' reach. Quite unreasonably, Fujifilm wanted its gear back afterwards, so later on I was shooting with my X-H1 and older 100-400 lens. I posted my thoughts and some images here on the forum (link below). I also have the 1.4 converter and agree that its use does not degrade image quality in any noticeable way. From what I understand, the converters are an integral part of the lens design. I have the 80 mm macro and the converters can be used with that as well which adds useful versatility.
Based on my short period of time with the new lens, I slightly prefer the results from 100-400. With the 1.4 converter attached, it has a simlar range of focal lengths and maximum apertures as the 150-600. To date, I have not felt the need to have a longer telephoto. One thing that I did notice, though, was how warm my black lens got in the sun compared to the 150-600 with its silver finish.
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