With testing many different shaders over dozens of games, the generated instruction count was down by about 2.5%, the code size around 2% lower, etc. Some users report this quantifying in some games like Cyberpunk 2077 yielding 4~14% better performance.
Very cool that Valve has developers contributing to Mesa.
Valve has a substantial interest in AMD linux performance considering they sell hardware that is both AMD and Linux (they did contribute before the steam deck, but now they have a concrete reason to)