Not sure if ATS Racing up in Lewisville has WBO2 or not. FIS in McKinney does. That's where my WRX was tuned with WBO2. Not sure if they'd be open to doing a Miata as the Scoobies are their bread and butter.
http://www.atsracing.net
http://www.fi-systems.com/contact/
Edit: According to their website, ATS does have a WBO2. I also know they have a bit of experience with Miatas.