its a band-aid for an inadequate fuel system or poor octane levels... its not a cure-all. its a finite source; when youre out of alky, youre out of boost... sort of. youll be limited to low boost. plus... it creates a possibility of burning a piston, like if your doing a run and your tank goes emtpy but you dont know it. its just creating a situation where stuff can go wrong. i wouldnt rely on it to make power. i would rely on adequate fuel system sizing, race gas or a bad ass intercooler setup.

you know when youre out of high octane gas, and with a datalogger, youll know when your fuel system is at its limits.

the octane-increasing benefit gained by using alcohol is marginal at best. in a 10% concentration, ethanol will raise octane by 2.5-3.0 units. 10% concentration is typically referred to as E-10 fuel, a reformulated gasoline which contains ethanol or MTBE as a oxygenate. it helps the gas burn cleaner to meet the Clean Air Act. the only problem is that the alcohol (Ethanol or MTBE) reacts w/ the water vapor in the air and will cause the fuel to separate into a gasoline layer and an alcohol/water layer. My mom was telling me about this and how it sucks for the Californians because the shelf-life is reduced w/ RFG's. when it separates out, the octane is reduced. the MTBE is Methanol (wood liquor) reacted with Ether and produces Methyl Tertiary Butyl Ether which is allowed in a 15% concentration over Ethanol (corn liquor).

the Main benefit of alky injection is the latent heat absorbed by the evaporation of the alky liquid. it reduces the intake charges substantially and will help control knock as 4drwhore already stated. but a properly sized and tuned fuel system will control knock quite welll on pump gas. running Toluene/Xylene additives will only reduce the knock tendancy more and allow more boost or cylinder pressures, thus a higher tolerance for heat.

tune a rich curve for street duty on pump gas and then tune a "lean" curve for track/weekend duty on xylene/race gas.

alky/water injection is just a crutch, to me. i would personally never put myself in a situation to rely on an item which has the possibilty of running out on me when i most need it, like nitrous on a slow car.