Aw man you guys got me going today....
Brooksie, how much do you drive a month? You then have to ask yourself the question whether it's worth it to save that $3 per tank of gas, for me $15 per month by using an unrecommended fuel in the motor. Where does that $15/month trade-off come from, one less time going out to dinner, one less steak, three lattes in the month? The real problem is knocking. Yeah, the ECU in the Civic can correct for it, but I know from experience with my old Nissan Pathfinder, it DID NOT like anything less than 91 (yeah, it required premium, gotta love that VQ35) with both bad gas mileage and knocking. Do that for a long time and you could cause your engine to die at an early age, or cause no problems whatsoever. But with the high compression ratio on the Si, I'd personally just try to figure out where that extra $10-15 per month on what is a $200 gas bill is going to come from.