The general consensus seems to be that the radiator performs best when given the coldest available air, i.e. amibent outside air, which with a top mounted rad will mean cold air from outside being sucked/blown through the rad such that hotter air enters the case. This is supposed to give you the lowest temps on your waterblock cooled components. However it is inevitable that everything which is not water cooled is going to suffer as the case ambient is going to be higher than if the rad fans were extracting air. Whether this is an issue depends on how sensitive your other gear is to higher internal ambient temps.
Me personally, I think this would be an issue for me as my graphics card water block only cools the core, the ram and vrms rely on heat sinks, plus I gues your HDD temps would go up as well. If your chipset is not water cooled then that would also be affected. At the moment both my rads are fully outside the case so it is not a problem, but I am seriously thinking about getting the Corsair 800D case and thus am currently trying to work out the best flow of air. Getting cold air into the 800D mobo area via the bottom fan seems to be the one weakness of this case but I have ideas.
