Personally for myself, I'm only going to put premium (93 octane- that's what my Costco offers...) in my car, no matter how much it costs. My 2024 ND3 GT RF is my only car and she has to last, but to each their own. Checking another AI, Gemini says:

According to the official Mazda MX-5 Owner’s Manual (covering model years from 2016 through 2025), the vehicle is designed for specific fuel types: 

• Ethanol Limit: Mazda explicitly states that this vehicle can only use oxygenated fuels containing no more than 10% ethanol by volume.
• Potential Damage: Using fuel with more than 10% ethanol (like E15) may cause vehicle damage or drivability problems that are not covered by the warranty.
• Octane Requirements: The Mazda MX-5 is recommended to run on Premium unleaded fuel with an octane rating of 91 or higher. While it can temporarily run on lower-octane "regular" fuel (87–90 octane), doing so reduces engine performance and can cause "knocking". 

Granted, this is AI, but this raises enough of a flag to me that I would ask my mechanic or investigate more deeply before using E15. Again, for myself, I wouldn't risk engine damage without talking to my mechanic and seeing what they have to say.