Cycles already uses specialized CPU kernels, tailored to several CPU instruction sets, for its processing. You won’t see much gain there by just building with new options.
You might see it with heavy geometry processing or particle systems though. Unfortunately things like the particle systems, cloth sims, and Mantaflow are end of life and not being maintained, improved, or fixed. Those are the types of areas that could see decent improvement but gains there probably don’t mean much any longer.
Maybe some boolean scenarios would improve? Maybe operations which force the recalculation of mesh normals in either edit or sculpt mode would see some benefit? Maybe texture painting becomes better? Maybe complex armatures animate faster? etc. etc.
The remaining areas that could show an improvement are harder to quantify and would require much more work to check. For instance all of the dependencies that Blender uses. Recently SSE was enabled for ffmpeg which yielded good results for VSE scenarios. Maybe OpenVDB scenarios for volume processing (not rendering) through geometry nodes would show some gain.
There needs to be a plan for what user-facing scenarios we’d check. The above is not the complete list.