So I had a go on it using Prey and also Tomb Raider. You can't get it to work on 8K as there is no 4.00X multiplier for DL Scaling.
What was weird however was even using 1.78x DL performance would tank a lot. Seemed same performance as Legacy if I was to take a guess based on percentages. I did to a clean install of Nvidia drivers though, so could be something that is required to make it work was not installed.
Nice feature, but I do not think I will be needing it for anything I am playing currently anyway and by the time I do it may improve and would be good if they offered a 4.00X DL option for we can test 8K
You can test at 8K using legacy DSR at X4 if your native res is 4k as long as you have the VRAM available as both the games you tried require 12.5+GB to run at 8k so best tried on a 3090 . The cost of using DLDSR is the same as using DSR but the performance increase comes from the fact you are getting better picture quality using DLDSR which is akin to the picture quality of DSR 4x but at the cost of DSR 2.25x, so 2.25x DSR and 2.25x DLDSR will have the same performance hit on the GPU but the DLDSR will be outputting a better picture than doesn't cost as much as 4x DSR