Reducing Luminance Noise does reduce image detail — it turns details like fur and feathers mushy.
My personal choice, when dealing with high ISO images, is to live with the Luminance Noise (reducing it by the minimal possible amount or not reduceing it at all) but to reduce Colour Noise and use the Colour Smoother slider too.
Shooting at these very high ISO settings is a more journalistic approach to photography and I am loving the new freedom that high ISO gives me to shoot at high shutter speeds and in near darkness.