We have had a tremendous amount of success using the MIata Manley Forged H Beam Rods in a lot of our builds and our customer builds. We have ran them over 30psi of boost and 8000rmp with even a little of nitrous on top. Manley Froged Rods are a great choice for your Miata especially for turbo applications. It will allow you to run more power reliably while strengthening a crucial engine internal component.
I personally have these rods in my car and have been pushing 392whp and 325trq for years.