How much FPS do you really gain

It depends on the hardware and the game. In benchmarks (Cinebench, 3DMark) — +3-7%. In games (CS2, Apex, Valorant) — +10-25 FPS on mid-range PCs and +40-60 FPS on weak ones.

On top-end hardware (RTX 4080+, i9) the gain is minimal (1-3%), because the bottleneck is the GPU/CPU, not Windows.