It's the OP's money and he's free to decide. Something to consider though - adding a bigger turbo (I know the IS20 is still small) and running low octane is perfectly feasible, but be aware you're trading some driveability. The turbo will spool slower and provide less torque without the advanced timing a higher octane will allow. You will also be relying entirely on boost pressure to make your power, so it will feel laggy, then lose power over 4500 rpm or so. Now, this is all relative, so it's probably nowhere near as bad as I just made it sound, but it's worth knowing before you decide. Best of luck with whatever you choose, I'm sure you'll enjoy it either way.