I tested the Sigma 150-600 last July on an RP and on my M50 body, and found it problematic. Severe focus hunting on both bodies, but worse on the RP. Subject was fast-moving racing boats similar to my profile pic. Frequently in servo focus, the lens would wander off the subject and pick something in the background or foreground to focus on for a few frames. Also run away chromatic aberration that was bad enough that I had trouble correcting it in Lightroom. And from about 225mm down - especially on the full frame camera - there was a lot of vignetting. That went away on the crop sensor camera except at the bottom end. To validate what I found, I rented a 100-400 and a 1.4x for another event in September, and the results were superior, even with the teleconverter on. For my part, I’ll take a pass and wait to see what this lens has to offer. I get that it costs more, but I’d rather save up and spend the money and get a lens that won’t frustrate me with its results.
To be fair, the Sigma 150-600’s worst faults can be corrected with careful post-production processing. But I’d rather spend my time shooting. I think a lot of it has to do with my being an older dog learning newer tricks, too. I have been shooting with a Canon SLR since high school, which was a LONG time ago. I am still programmed to shoot with the final results in mind, not what I can correct in post. So I still look at extensive precessing work as correcting what I did wrong rather than enhancing things. That part of my thinking hasn’t changed with the times, and that makes me part of the weak spot.