If you're careful with your tuning, you can run 11.8:1 on 90 octane gas. Just take extra care with your timing maps. Toyota sells the Celica GT-S with an 11.5:1 compression ratio, and that runs on 91 octane with a street tune conservative enough to survive if a retarded owner puts in 87. Tune yours well and you can get it run right on 90 and still make decent power. Of course, your power potential would be much higher with higher octane gas, but it will be difficult for you to make sure every tank of gas is consistent while mixing in octane booster.