excelent photo Cristophe. I think the sky is all right. many times happend to me the same conditions -almost pure white highlights and very dark clouds. I always try to balance it as much as possible with my nd grads or even in PS but thats just the way it is. you can't do more then this about it.
Not too sure about that large, white, exposed part of the sky, half of it seems dark, and the other almost pure white... not sure what's going on there, whether it was down to a filter used, or the post processing. Maybe a tad too sharp as Drew, and someone else said, but overall, a very nicely composed, and lit shot.