Based on the NVIDIA's claimed performance uplift in Cyberpunk 2077 Overdrive mode with 4x FG and Alan Wake 2 Full RT w/ 4 x FG and Digital Foundry's reporting that you see a 70% increase in FPS moving from 2x to 4x FG,, and what we know of the performance of the 4080(S) and 4090 in these games, the 4090 will pretty easily beat the 5080 when using 2x FG in these path-traced titles, and the 5090 should beat the 5080 by a 55-60%+ margin when both are compared with 4x FG. Nvidia's first-party benchmarks show the 5090 achieving 2.33-2.41x scaling versus the 4090 (4x versus 2x FG), whereas the 5080 only shows 2-2.04x scaling versus the 4080 at the same settings.

As an example, we already know that AW2 is around 31% faster at 4K DLSS Performance + FG. Daniel Owen's benchmark shows the 4090 at around 105 FPS versus 80 for the 4080 Super. NVIDIA shows that the 5090 with 4x FG achieves 2.41x scaling, which is around 253 FPS. NVIDIA also had a DLSS4 presentation at CES showing AW2 at DLSS 4K Performance mode with Ray Reconstruction using the new Transformer model + 4x FG, with a framerate monitor, that showed high 200s to low 300s FPS in an indoor scene, so a 253 FPS average including more difficult outdoor content is reasonable. In contrast, the 5080 only claims a 2.04x scaling, so 163 FPS. 253/163 = 55% higher performance for the 5090. However, when you back out the gains from 4x FG, you're down to around 94 FPS at 2x FG versus 105 on the 4090, so the 4090 still retains a 12% advantage.

AW2 and CP2077's path-tracing modes are some of the most demanding games on PC, so this doesn't necessarily represent performance scaling for pure raster titles or even lighter RT games. Still, it's arguably in path-tracing games like this where raw performance is needed the most, since you don't want to use FG from a low base, or have to use excessive upscaling. So, it's relevant that these extremely demanding titles are likely to still perform better on a 4090 than 5080 when using 2x FG or no FG. The new Transformer model does appear to provide huge improvements to temporal stability and detail, particularly as to ray reconstruction, but those benefits will also apply to the 4090.