You don't have to wait til GTA6. GTA6 running at 30 or 60 fps won't prove anything about Digital Foundry that can't be proved today.
Digital Foundry slobbered over the Switch OLED and the Steam Deck OLED despite few if any technical improvements. Their decision to slander the PS5 Pro, especially this early without knowing how effective PSSR and the raytracing features, tells you everything you need to know about them.
It's not really how it works.
Cerny essentially would have a very limited budget. If you say Oh Hi Mark, you have 100 dollars more than current production costs for the PS5 Pro, he can't simply say okay, for 100 dollars I can get GTA6 running at 60 FPS.
The PS5 Pro is probably a wishlist item for Cerny and surveyed developers both internally and externally that couldn't be put in the PS5 at the time of its launch combined with Sony's investments in AI with iSize and Audeze.
I actually think PSSR will be a bigger deal for the PS6, Cloud, and the PlayStation handheld than it will be for the PS5 Pro.
I think that is why Sony has invested in their own AI upscaling rather than just rely on FSR or even AMD's AI enhancements in the future. Unlike DLSS 3, Sony is suggesting that all games can support PSSR without specific datasets. If that's true, it's major breakthrough for AI upscaling.
This will be a major success for running all games on a PlayStation handheld and running them in the cloud at lower resolutions.