I have the feeling that some hairstyles doesn't render properly in P6. For instance Shag Hair or Wild Hair. It's looking clumpy and not detailled at all. Any ideas or even solutions?

Common things that help are :-

Lower the shading rate
Turn off texture filtering
Downsize the transmaps

Experiment with various combinations to see what works best for each particular hair model / texture set.

Sassy Hair is another one causes problems, with weird arifacts in the hair

Some older hair models may also need the "Add a small displacement" fix (those with double sided polygons).
It usually fairly obvious when this is needed as you get a blocky appearance.

Something else to consider is to check the "Normals Forward" option in the Material Room. I had a problem with the new version of April Spirals until I did this. April YSH has now updated the hair so you don't need to do this manually but older hair styles may benefit.

Wild hair renders ok with me in firefly look at my last gallery upload and u will see