Well, it seems to me there's two ways to take advantage of the effective octane bump from using water injection on a NA:
1) Increase compression, therefore boosting engine efficiency across the band. Too high, and if you run out of water you'll be stranded because your engine will detonate to death.
2) Leaner burn at cruise, therefore boosting efficiency on the highway, possibly alot. Prolly wouldn't work as well in town since in town driving is best done pulse and glide (for me anyways.) Although it would be nice to tune out WOT fuel enrichment and replace it with water... which would help in town efficiency a lil bit.
THe problem with using WI for lean burn is the method of turn on, and tuning that little fluid. There are guys literally running 40 gph of methanol and water. I am looking at 0.5-1.5 gph using a MAP sensor based progressive kit. If there was a trigger that said "You are in lean burn", then a nozzle could be activated, but from what I have read, lean burn kicks on and off, and I think that there would have to be a very small amount injected to manage the lack of fuel being used.
I can't remember who, but someone made a direct injection 6 stroke engine, It would go compression, power, exhaust?, water/power/cooling, exhaust, intake.
The big problem they had was with injecting this much water into the engine, the oil would get contaminated, the cylinder walls would also get partially steam cleaned (no lube). I always found it to be an interesting little engineering experiment.