My guess is, based on experience and intuition, that acetone mixed with gasoline will degrade the fuel lines faster than gas alone. The acetone won't really be affected terribly in its destructive properties toward your hose by the gas other than to serve as a dilutent so it will be slower than pure acetone. Why it does this I'm not exactly sure, but ketones are notorious for swelling polymers, leaching out plasticizers, things like that. It may be doing some of these things.
It is true that a lower flash point fuel will improve fuel economy; this is why cheaper grade gasoline gives you better gas mileage--it's got a lower flash point. However, you can also get 'pinging' in the engine--pre-ignition sparking coming from the gas igniting too early. This isn't good for your car. In addition, if you have a high-performance car, the compression in the cylinders REQUIRES a better grade fuel with a higher flash point to run correctly.
Also remember that acetone is about 5% water, something you don't want to put in your engine. I doubt there is an interaction between acetone and gas that will damage your fuel lines, but you probably don't want to add acetone for the following reasons:
1. Pre-ignition of the fuel
2. Damage to fuel lines from normal acetone properties
3. Water in acetone condenses inside cylinders, or rusts out your gas tank
4. Ketones and other oxygen-bearing organics like ethanol have been known to produce carcinogenic incomplete combustion products in exhaust.