Water injection has mostly been used to suppress detonation. IIRC, injecting equal parts water and fuel raises effective octane 8 points. There's a paper out there I calculated this from. That isn't this threads topic, though.
I first thought of using water to aid air cooled engine cooling at high power levels some years ago. It didn't take me long to find three papers on the subject. Here's one.
"Based on this work, several pertinent recommendations have been made: (1) utilize water
injection for short-duration, very high-output operation which would otherwise be destructive
due to thermal overload"
Yep, exactly what I had in mind! This particular study was done on diesel engines to investigate total cooling from water injection. Port injection was found to need a 3.36 ratio of water to fuel for complete engine cooling. The actual numbers will vary a bit for SI. As a guess, we may need 4:1 for complete cooling.
Now, if I'm hosing in water at high power levels, I might as well take advantage of the effective octane boost as well and raise the compression. I'll have a more efficient engine at part throttle cruise that doesn't need water. A more powerful engine at full throttle, that does need water. Other than added complexity, it's all win. The injected water substitutes for the rich mixture traditionally used, so offsets the system weight.
I'm wondering if I could make a water 'carb'. It may be hard getting a fine enough spray.
Meth/water could be used too, but then it becomes fuel, which complicates dosing. It's going to be far harder to top up with meth/water at the average airfield. Water tends to easier to find than fuel...