Air Extractor: Better than a Plastic Jesus?

Noticing all the openings behind the cockpits of mainly Schleicher gliders such as -27 and 29's I'm asking myself if this is a real drag reducer or a placebo. For those unfamiliar: people have been cutting 4x2 openings into the fuselage about 8" behind the canopy cutout and put air vents - some with funnel-like contraptions on the inside of the turtle deck into their gliders. The funnel seems to suggest to the air molecules "this way out", some pilots may put arrow stickers on the inside so the air knows which way to flow. The whole thing is meant to avoid pressurizing the cockpit from the front air vent. Air leaking from the canopy seal would trip outside air flow to become turbulent.

Are there any serious comparison flights that were done following a scientific protocol to show the effect of such devices? Is this really better than putting a Tibetan prayer flag on the tail or a plastic Jesus on the glare shield? The latter can serve double purpose since I know where I would stick my transponder or PFlarm antenna...
Seriously, I'd be interested if anyone has found this thing useful beyond the statement that it must be good since all these hotshot pilots (insert call signs here) have one.

I'm waiting with jigsaw in hand for your responses, the LS8 rests uneasily in the basement!
Herb, J7