This is a really difficult question, and it changes person by person. But the answer really boils down to: when you are very current on your canopy and looking for more performance.
And when a coach or very experienced pilot who has watched you fly and land, advises it. It could also be an option if you don't feel ready to downsize but want to increase the speed of your current canopy choice.
You also have the option of just using the removable slider part of the RDS (what is the difference?), which is a good first step.