I am no mechanic but i thought the turbo was spinning all the time it just works harder (spins more) the more you push it.
Re dash light, when would it come on, it does not know you are nearing the end of your journey or it would only come on when the car came to a stop (so at every traffic light) and there are temp parameters in the stop start process but what you are doing is hopefully increasing the life span of the Turbo
This
thread sums up why
Your turbo heats up to around 900 degrees Celsius
under hard driving conditions. When you shut off the car, You suddenly take away the heat source and the ambient temperature falls to around 95 degrees Celsius (the normal operating temperature for a Subaru that has been driven hard). This causes the turbine housing to experience thermal shock. Now granted, it would take a long time for this thermal shock to actually cause damage, but you're probably reducing the turbo's life by putting it under stress unnecessarily. If you let the car idle for a minute or so, you allow the exhaust gas temperatures to gradually go down to 700, 600, 500, etc.
An example to support what I said: 1. When a cast iron manifold is welded (turbine housing is cast iron too), it is recommended to toss the whole thing into a fire and let it burn out overnight so that the manifold cools down gradually to prevent stress points from forming.
From experience,
it's not necessary to let the car idle if you simply drive normally for the last 2 or 3 miles before you shut off the engine. Just never shut the car off
immediately after doing a hard pull. E.g. when the cops pull you over on the highway.