Forge themselves powder coated it I asked them to paint it black not powder coat it, they (John) claimed it wouldn't effect as he put it the "thermal efficiency" or warranty.
I'll might have to ask them about this bearing in mind what you say. 
I paid £650 all inc plus £50 for the powder coating.
With regards the S3 cooler, I'll see how I get on with this maybe I'll need one with all the powder on it 
I reckon John (or you!
) might have confused the powder coating vs paint. When I initially spoke to Russell from Forge, he carefully explained to me that they will only flash spray a very thin coat of black paint on their coolers - and when they sent mine, the paint was very thin - but is standing up well to the winter without a single chip. Maybe you ought to speak to Russell. 
Ok just had a phone conversation with John at Forge:
Here are the main points discussed:
1) The cooler is powder coated. Russel at Forge speciifes that powder coating is to be a min of 0.1mm and a maximum of 0.15mm.
2) The powder coating is offered in place of the spraying as a) it gives more of a uniform covering and b) it will not flake.
3) Forge (Chris) has run back to back tests with powder coated and non powder coated coolers and found no signifcant effect on the cooling. (less than 1 degree c loss)
4) Anodising was considered but not chosen as it would have weakened the weave of the aluminium.