Mine made immediately measurable gains (SG1) of about 3% @ 95 km/h @ 2C. http://www.metrompg.com/posts/grille...ing-part-2.htm
. If you're not directly measuring with instrumentation, the effect will likely be hidden in tank-to-tank variability.
I finally made my "good" version (after at least 4 cardboard & duct tape versions) last weekend: a custom "bra" - helps keep the bugs off too! I'll post a pic eventually.
Is your hood fully closed in that pic?