In having this conversation with Tim at The Fab Shop, he found that his design of upswept headers actually put up better dyno numbers. I don't believe there was a significant amount for everyone to start selling off downswept headers.
Personally, the only reason we ever ran upswept headers was for the space needed when hauling 2 dragsters in the trailer. I like the looks of the downswept header and they are out of the way of getting to the nitrous bottle and the dial board. Like David mentioned, they are also nice when working on the top half of the car or taking out the trans.