It's more than just compression, it's the timing of the motor that makes use of lower octane fuel risky. You run the risk of detonation which will tear up your top end. Regardless of how 'fine' you think it's running, I'll bet that if you accelerate under full throttle or up a steep hill you will get detonation. The car has detonation sensors which may attempt to retard timing enough to cure the condition, but by then you will have already been detonating in your cylinders possibly damaging them. And your performance will suffer to boot.

I honestly can't see that saving $3 on a tank of gas is worth that risk.