I can provide a little info here, providing that we are talking about the same thing. By "windbreaker," I am talking about the plastic shield that you install behind the two seats that reduces wind turbulence in the cockpit.
This device appeared early after the Miata appeared; it was one of the first after-market items for the Miata. The down side: IT ONLY WORKS EFFECTIVELY WITH THE WINDOWS ROLLED UP !!
I was at a Car Show last weekend sponsored by a company that sells these windbreakers. I asked one of the company owners if this is still the case. He said "yes." Miata Magazine would report that the shield was about 25% effective with the windows down, and about 90% effective with the windows up. I passed on the shield, as I cannot bear to drive a convertible with the top down and the windows up or sidecurtains on.
FWIW, I hope this helps.