I think the CYA curve flattens pretty quickly at around 20ppm as far as chlorine retention in sunlight goes so there are few benefits to going too much higher. In fact, something like 80% of chlorine retention is achieved at the 10-12 ppm CYA level.There are however three really big negatives to going much higher than about 30-40 ppm CYA:
1) The higher the CYA value the less "active chlorine" there is available at the same free chlorine level and it's "active chlorine" and not free chlorine that kills.
Example: At a CYA of 0.0 ppm (no CYA in pool), and say, 4 ppm free chlorine, you have a full 4 ppm of "active chlorine" (chlorine with the ability to kill) but little reserve because there is no CYA to stabilize or to "bank" the chlorine. At 20ppm CYA and the same 4 ppm chlorine, your active chlorine might be only 0.1 ppm and that's not a typo. This is because at 20 ppm CYA, something like (ballpark) 95% of the free chlorine is bound to the CYA and of the remaining 5%, only half of that or 2.5% has effective killing power (assumes Ph around 7.5). This means that 2.5% of the initial 4 ppm chlorine (or about 0.1 ppm) is available as active chlorine and the rest is "banked" by the CYA. The CYA will release more chlorine but only after the front line chlorine (active chlorine) is depleted. Fortunately, only about 0.06-0.15 ppm active chlorine is required to kill more than half of the germs so 20 ppm CYA is a "nice spot" to be at since it shields chlorine from sun, has reasonable kill times and good Oxidation Reduction Potentials (ORP's).
2) Increasing the CYA levels increases substantially the time required for chlorine to kill. With no CYA I think the chlorine kill times are less than a few seconds. At CYA levels of 30-80 ppm the kill times are (ballpark) several minutes to an hour or more.
3) And finally, the oxidation reduction potential drops as CYA increases, and flattens at about 70 ppm CYA. Add as much chlorine as you like and beyond about 70ppm CYA the active chlorine tops out at about 0.18-0.2 ppm. This level is sufficient to kill but kill times are VERY long, ORP's are low and a LOT of chlorine must be added (about 14 ppm free chlorine to obtain 0.1 ppm active chlorine, something like that). Compare this to 20 ppm CYA where you only need 4 ppm free chlorine to get this same 0.1 ppm active chlorine and you get much faster kill times and higher ORP's.
So here's where people get into trouble: they use stabilized chlorine and with each application they increase their CYA levels and don't increase their free chlorine to compensate for rising CYA levels. When CYA levels finally get too high, virtually all of the chlorine is bound to the CYA and very little active chlorine is available. If you keep chlorine levels constant and let CYA continue to climb, the active chlorine decreases to the point where "bad things" multiply faster than you can kill them. With little active chlorine and an ever increasing number of bacteria and algae, all the active chlorine is consumed thus causing the CYA to release more and more chlorine to replenish the lost active chlorine, thus depleting the chlorine bank. Your test kit (when you measure free chlorine) will show free chlorine plummeting because it is consuming an ever increasing amount of bacteria. Shortly after this happens your free chlorine goes to zero and you begin to see cloudy/green water appear. The solution is to drain water, refill to lower CYA levels to about 10-40 and to only use unstabilized chlorine (sodium hypochlorite, calcium hypochlorite or lithium hypochlorite) from then on. The 10-40 ppm CYA range is enough to protect chlorine from the sun while still providing a good chlorine "bank" and high enough active chlorine levels, reasonably fast kill times and good oxidation reduction potential levels.