The Screen wash supply is the same supply for the Intercooler spray.
Using the spary when the outside temperature is cold will make no difference.
The theory behind the spray is that cool air is denser than hot air, so the cooler the air the 'more' air you can get into the engine. The Inercoolers job is to cool the air, so if the outside temperatue is HOT then sprayin cold water over the intercoor should cool the air down more than if the spray was not there.
I personally think that there are a few drawbacks with the system, the first one is that there will always be a time lag from when you press the button - to the water spraying - to the air in the intercooler getting into the engine and being burnt. This means that you need to plan ahead. If you are busy conentrating, (usually going round a bend at the time) finding the button to press isn't the easiest thing to do. It will probably be of benefit it it is like on the rally cars and operated done automatically when a specific boost is met (or whatever) and is basically left spraying. I do not know how long the washer bottle would last far, but I feel that with our poor weather and road conditions the water is best used on the windscreen.
Regards
Markja