I borrowed that tonneau cover from a Miata-owning friend this summer with the intention of tuft testing it to see how it affects airflow.
Although I used it regularly, I never quite got around to doing the test.
But I'm pretty confident it helps somewhat, just based on the fact that there's noticeably less wind noise with the tonneau on than off.
With the tonneau off, these open areas at the sides
seem to be where the wind noise is coming from:
I'd still like to tuft it, though. I haven't found any info about the aero effectiveness of the Miata in several searches. It'd be nice to contribute some new info to the Miata world.
But I'm about to quit driving the car for this year. MPG on the current tank is beginning to suffer from the colder weather - it's around 47 near the 3/4 mark.