The modder released it (
Starfield UpScaler for those that want to use it) after I posted.
Alas, it is more half DLSS, as although the graphics look slightly better, no one has found any performance boosts from it compared to normal native DLSS.
I installed it yesterday and played a bit and the biggest improvements seem to be occlusion rendering and the included ReShader helping with textures and pop-in. I am running on Ultra on a 3050ti (8 GB), performance overclock, high TP SSD, with Starfield realtime thread priority on my i5-12500H (16 GB DDR5), and am seeing solid 45 FPS, with drop down to 30 FPS in places, and 60 FPS when in non-taxing interiors. I may drop it to High and 67% Resolution Render (which becomes 50% Performance Balance when using the DLSS mod) to see if I can get a consistent 60 FPS.
It is definitely a Bethesda game in that it isn’t optimised well (Creation engine makes that very difficult), made worse by their exclusive deal with AMD, which doesn’t make much sense for their market, given ~75% of those playing Starfield will be on Nvidia GPUs and native DLSS3.5 would work wonders for game performance.
Still, better than nothing, I suppose.