Centaurus you've hit on an interesting an often misunderstood issue. The amount of increase in boost the turbo delivers at even 1000rpm (versus say 650rpm) is almost nothing (certainly in the region of less than half an inch MAP due lag and blade efficiency). There would of course be an increase in cooling airflow through both sides of the turbo at 1000rpm and a corresponding increase in oil pressure (therefore cooling oil flow through the turbo) at the higher idle rpm.
The issue isn't to remove all of the temperature from the unit, it is to stabilise the temps to a more even (read: no hotspots) temperature within the unit. The idea is to remove the workload from the engine enough to temp stabilise it to a dull roar. Some POH's for the aircraft also make mention of EGT's to be below a certain temperature also before shutdown in addition to the time requirement. Sustained lower rpm may also lead to fouling.
Alternators also rely upon more RPM to provide a higher amperage so satisfy any current draw load that the pilot has set them the task of satisfying. Personally, I wouldn't use 1200rpm to taxi at night unless I was climbing a steepish taxiway such as at Darwin. Have flown the chief (albeit a while ago), but have many more hours in the C402 so cannot really comment on the 1200rpm except to say that that if I were paying the bills I wouldn't be keen on it.
OpsN.
