Is the car your primary car? I know sydney dont have a lot of E85 stations, so is there one close to you?

For N/A stock comp I wouldn't bother at all, the availability of 98 just outweighs the pros in performance (in sydney)

If it were melbourne where theres E85 in every second suburb, would just be a personal choice.

Even if you were to do 12.5cr k24, 98 is still fine. Lol