I found a pretty good article.
http://www.pugetsystems.com/labs/articles/Vertical-vs-Horizontal-Case-Cooling-89/
They're measuring overall case cooling performance in regard to horizontal vs vertical, but they also record gpu temps - gpu's which are using heat pipes. No change whatsoever.
It's not that I don't believe gravity can affect heat pipes, but maybe heatpipes on graphics cards are just too short to really show any difference. Anyways, this is good for me to know for future video card selection.
edit: although looking at their vertical orientation, the heatpipes of the gpu are still technically horizontal, and not in a "bad orientation" as depicted by your SilverStone link. I think I've been worried for nothing.