Yes, if what you are talking about is water injection, I have it from a reliable source (engineer who works in the automotive industry) that just straight up water injection will give you a 10-15% boost in FE.
Came across this thread and had an idea, how hard would it be to convert a 4 cycle into an 8 cycle. Instead of having to go with a new engine. It would be less efficient but may still be worth while granted probably with some HP loss.
My idea is to have two injectors per cylinder, one for water and one for Gasoline. Every other "Power stroke" would use water and the other use the conventional power stroke. Would there be too much lost efficiency to gain anything from this conversion?
Makes me want to set up a test bed with an old lawn mower engine and some way to test the HP it puts out.