Insomniac's interview from PSBlog made it seem like they've tried something different with HDR:
"We've tried to find the right balance with our HDR implementation such that the medium tones and dark tones in the image look the same as they would in non-HDR, but the brightest parts of the image really pop."
Does this matchup with your findings so far?
So what I've finding is that for the bulk of gameplay the image sits in a very similar range of luminance to SDR content, VERY bright in game objects such as the Sun and explosions seem to peak at approx 700 nits and of course this is seen in lower values in specular highlights.
I've caught a couple of pixels in the centre of particle effects reaching higher values, but these have been super rare.
This is often the way that many UHD 4K discs are mastered, as either the content itself was created with HDR in mind or the HDR master of the disc is deliberately trying not to deviate from the directors SDR vision for the movie.
Blade Runner 2049 is an example of this as well as many older films that have had 4K releases
It may be that later on in the game, when certain other effects such as electricity become more visible, that we see more of these higher values.
On the plus side, the fairly low dynamic range that the main game is using will give a very familiar filmic vibe, which may well be intentional given the type of game it is.
The game also readily outputs near black data, not raising the blacks and tinting them to give them teal movie shadows.
The main issue that this approach will give for some users is that certain common models of HDR TVs are expecting videogame content to be 4000nits, so when presented with content such as this that has significantly lower peak brightness code values, the TV will hold back , reserving it's peak brightness output for high values of data, which of course it never receives, resulting in a dim image.
No game to data to date has produced metadata to stop this happening, however I will check this later too, in case Sony have open up some extra tools in the SDK.