To get more details from your hair textures decrease the setting for "minimum shading rate" to 
a value between 0.2 and 0.5.
This will increase render times but give much more detailed render.
At the same time you should uncheck "visible in raytracing" for the hair.
That will keep the render times from going thru the roof :)

Another thing you may consider is increasing the "max texture size" to 2048 or even 4096.

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