It seems to me if one set of compounds boosts octane (and I thought that toluene was added to fuel for that very purpose) or the other would NOT boost octane. But I could be wrong, and would like to be corrected by anyone "in the know".
Also - I did believe thaat water injection does boost cylinder pressures (as a result of the vapor converting to steam) and improve performance, but only in a properly engineered injection system.
I think your right about the diesel being unlikely to help. Diesel has a lower octane rating than petrol although octane figures for diesel are not much use.
No idea if a small amount of diesel in petrol would help in any other way, but a stock f or 2F shouldn't need high octane fuel, perhaps on a very worn engine it might aid starting, who knows.
Octane figure is not the amount of energy stored in the fuel etc, it's the resistance of the fuel to auto-ignite on compression, the higher the number the less likely the fuel is to ignite before the spark ignition.
The diesel cycle needs a fuel that ignites on compression. Cetane index is used to determine the ignition characteristics for diesel fuels and the gaol is the reverse of petrol.
Water injection is usally used to cool the air charge in a boosted engine, very important if detonation is to be prevented. It is true that as the water injection is turned on the need for fuel is often reduced and the injector cycle times can be less for the same power output - this is most likely because the a/f ratio is altered to closer to 14:1, before the water injection came on it might have been 10:1 with the extra fuel passing unburnt through the combustion process having been used only to cool down the air charge. The added water can make the use of a wideband sensor for a/f measurements more difficult. Important thng to rember is fuel can only be burnt is there is air avaliable for combustion.
That said people with more practical experience (and or) more chemistry than me have written that the water is broken down and the hydrogen and oxygen burnt, or the water becoming steam raises the MEP - mean effective pressure, and so power output - if it were that easy we would all have water injection etc....
