I have seen it documented time and again on other vehicle boards. The increase is primarily due to the ram air effect, though there is a small amount of cold air gain. Here is a link of a way to use a conical filter in conjunction with a snorkel. I have posted it here before, but it probably dropped off over time.

http://www.dpgoffroad.com/tech/default.htm

I have also seen it run to the fresh air intake for cooler, higher air verses a snorkel. The ram air wouldn't be there, but if you do not want the permanence, and expense of a snorkle this is a good compromose.