I'm not sure of specific figures but have seen various threads on various forums with data taken plus a few vids on Youtube with details aswell. It is a marked increase though as you get both heat soak from the turbo (the hotside of which can get very hot and well into the 800 degrees c range) as well as the compression of the air itself. At the end of the day it's down to the temperature of the air/fuel mix going into the cylinders - the colder that is, the denser it is and so the more oxygen it has to burn thus increased power. When people feel their turbo car is quicker in cold weather, it's not because the temperature of the air being sucked in is colder, but it makes the cooling efficiency of the intercooler better. The result is the same, just need to understand where in the chain it happens.
Any cold air intake mods on a turbo car should be taken with a pinch of salt. Plonk in a larger FMIC and you're on the right track
On an n/a vehicle though, cold air intakes are more important as that pretty much is going to be the temp of the air hitting the cylinders (barring heat soak in the intake system and so on).