Storms:
The effect of CYA above do 50 parts does not really change the effect on Chlorine... Remember for every lb of Tri or Di Chlor you are adding one half pound of CYA...
In which way?
Cyanuric acid has a huge effect on the behavior and effectiveness of chlorine. Anything over 100 is very alarming and personally I don't like to see it over 50. I am not sure if your statement suggest that once it hits 50 it stops having any effect whatsoever, but it really ties up the chlorine and decreases it's effectiveness.
When you deal with a lot of residential pools, you monitor the FAC
levels, which is typically acceptable, but you still have no idea how
effective your chlorine is, just how much of it is in the water.
When you work a lot with ORP controllers you get very familiar with the relationship between chlorine and the cya. We commonly see levels of 5ppm of chlorine having the same ORP @ 100ppm of CYA, as pools with 2ppm @25ppm.
Here is a decent article on the subject/
http://www.poolspanews.com/2008/052/052acid.html
Personally any level above 100ppm I would correct, and at 320ppm I would consider replacing all of the water.