OP might be a bit confused by now. This is the dynamic range chart in the camera manual, page 50:
- Pocket 6K dynamic range.jpg (403.08 KiB) Viewed 1107 times
It shows how many stops of dynamic range is above and below middle gray. At ISO 1000 there's 7.3 stops above middle gray vs. 3.9 stops above ISO 100. That's what Ellory means by shooting at ISO 1000 for more highlight retention. But this only applies if you're exposing for middle gray with shutter, aperture and lights. And it does bring noise levels up a lot. You would get the same result by selecting ISO 250, underexposing two stops and bringing the middle gray back up in post.
Since OP has problems bringing light levels down in the first place, using ISO 1000 is not going to help for that. Neutral density filters are the solution when lights can't be controlled. 2, 4 and 6 stop ND filters will cover most needs.
The chart also shows that within each ISO range 100…1000 and 1250…6400 the black and white clipping points are the same. That is, if you have the same exposure (shutter speed, aperture) it doesn't matter if your ISO is set as 100, 400 or even 1000 the white point will clip at the same light intensity. ISO is only metadata that can be changed in post (if you're shooting braw).