I know this is for the older C20LET but non-the-less a very interesting read. Taken from:
Turbo timersTotal waste of space.
Turbo timers are a remnant from days past, when all turbos were aircooled and synthetic oils were not around (or too expensive!)
Switching off immediately after a full-boost run would then cause the oil pump to stop, and the oil remaining in the turbo bearing would carbonise and turn to a form of ash (coking was the term)
Nowadays this cannot happen. The water jacket absorbs all the extra heat from the bearing, and then some. Therefore the oil temps don't exceed 120C even then, and synthetics tend to resist death until over 150C, so there is no benefit from the turbo timer, except unnecessary idling (that does the turbo seals no good, neither the camshafts)
Idle is not good for turbos, water cooled turbos don't need any of this maintenance. Just roll smoothly after a full-boost run, if you're going to switch off immediately, that's all."