Speed cowls certainly look like they'd make a huge difference, don't they.

Years ago a buddy and I added one to a very fast C/L rat racer. We were positive we'd get some major speed boost out of it. What we wound up getting that was obvious and significant was fuel economy. It came from the engine running cooler. No lie. The speed increase wasn't even enough that we knew for a fact it was faster. I was recording temp/pressure/humidity at the time and knew the speeds probably weren't worse, but they sure weren't a lot faster. So I figured I'd screwed up the proportions or something and tore the cowl off and did a more exact one. Same deal.

One very obvious improvement was our pit stops. Rat racers have to pit. The pilot shuts the engine off and lands. Pit guy refuels and restarts. You better get a one hit restart or you're toast. With the cowls, every pit was a one-flip restart. And for sure, we were using less fuel. And we think, but couldn't prove, the engines lasted better. Cooling does all that. But speed.......... We were sure the plane was faster, but were absolutely sure it wasn't much.

C/L is absolutely the best testing environment any amateur team could have. And I'd bet big money those Cd numbers are very good indications of what the cowl does for drag.