Using lower octane gas will make the electronics retard the spark which will cost you MPG, so you may not end up saving anything and might actually lose money and also lose a good bit of performance. Also retarded timing makes the engine run hotter, especially the exhaust side so you'd be risking damaging the exhaust valves and the manifolds etc. They say you "can" use lower octane as in IF you can't get the recomended grade. They never say you can run whatever you want to buy or whichever is cheapest. Seems dumb to risk damaging a $8,000+ engine trying to save $0.20 a gallon.
I always wonder why people buy hipo cars and then want to use cheap gas in them.. Is $3 a tank really that big a problem for you? You can afford a $30,000+ car buy can't spring for premium fuel? Just seems odd to me is all.