Super common in the vw/ audi world to add a gallon or two of e85 with every fill up of 91. Did that in my s3 for almost 7 years. Helps big time with preventing knock and allows ignition/timing to increase which give more power. Goal was e25-30. Been searching the Mazda forums for info and came across few post but looks positive ..but a stock 2.5t could benefit from a e25 mix imo especially with the high compression. The ecu can add or decrease power 87 octane or 91 here in California. So adding say 12 gallons of 91 and 2 of e85 your getting 93 octane and a e25 mix ..
Ill be trying my first fill up 😎