Hey,
Interesting combo there. The water/alcohol injection systems are usually most beneficial when at or near WOT. Just cruising around, it is not necessary (or probably desireable) to have water / alcohol spraying into your intake. It would cool the charge I suppose, but the mount of air going into the engine woudl be much less than at WOT. That being said, the ratio of water/alcohol to air/fuel would be such that the motor may not run very well. Moreover, you didn't say that your motor was boosted at all, so the incoming air would already be at ambient temp, so I'm not sure that trying to cool the air would be much benefit to you.
The only NA applications that benefit from water/alcohol injection to my knowledge are those with static compression ratios that are too great for WOT on pump gas (and then they have detonation problems from extremely high cylinder pressures). In that case, you could probably inject water / alcohol and get away with running pump gas on a 12:1 or better motor, that would normally be run on race gas. You didn't say what the compression ratio of your motor is.
As far as getting more mileage, I don't believe that water/alcohol injection is going to get you there by itself.
Working together with propane - I dunno what that would do. But, water/alcohol injection is not known for creating power by itself - what it allows is higher boost pressures, more timing (less retard anyway), to make more power under safer conditions (usually on a boosted motor, though the high compression motor could be an example as well).
The octane rating of the fuel overall would go up using gas plus the alcohol injection, especially if you injected straight methanol - but that's not really recommended due to extreme flammability, plus its a little harder to get a hold of. Just having higher octane fuel does not necessarily get you more power or more mileage - it just allows you to tune things perhaps more aggresively, which may yield some improvements depending on the engine combination.
Your motor sounds like it makes real good power already, maybe go forced induction if you want more. It really is the stuff. Throw a turbo or a centrifugal SC and have some fun - then you can definetely make use of water/alcohol injection.
The only way I know to get more power AND more gas mileage is with a turbocharged 4 or 6 cylinder, like you see in some of the newer cars. You get the gas mileage of the 4 cyl, plus the snort of a v-6 (or v-8 in some cases) when the boost hits.
