http://paraglidingforum.com/viewtopic.php?t=24213&start=8

Harness manufacturers keep optimizing pilot drag but how much of
the drag reduction is due to the pilot position and how much is due to the form
factor? How much back protection must we sacrifice in order to further reduce
drag? In the end, how much do we really gain?

Recent tests in the wind tunnel claim a forty six percent reduction of
pilot/harness drag using a state of the art supine harness over a "traditional"
harness with the pilot in seated position. No details about how much of this
gain is obtained by reducing front area and how much is due to the optimized
form factor.

We can't measure the form factor component without a wind tunnel but we can
easily measure the front area to get an idea of what's going on because form
factor and front area have the same weight in the formula of drag.