....."To get around these problems, the vast majority of heat pipes, including all of the pipes used for PC cooling, have a wick around the inside of the tube that draws the condensed fluid back to the hot end by capillary action. Wicked tubes don't work quite as well when they're hot-side-down as unwicked tubes, all other things being equal, and they also still work marginally worse when they're upside down. But that difference is nothing to worry about; it's likely to be quite difficult to even measure, if all you've got to do it with is motherboard temperature sensors......"
Assuming the article is correct, there should only be a marginal performance gain from mounting vertical.
Would be interesting to see the tests you've mentioned though.