Hissing noise is normal on hydraulic PS. Some cars have electronic PS, and those do not make any noise at full lock. My noise is a squeal, not a hiss (although the hiss is also present, which is normal)
My steering is exactly the same as it was the first day i bought the car - same noise, no worse, no better, although the dealer "fixed" it twice under warranty, thank you very much.
Sometimes, usually when very hot outside, it doesn't do the noise at all. I'm quite sure it's the belt slipping on the PS pulley when PS pump is under load (full lock or turning or very adherent surface)