Actually, according to the German test reports I've seen, the power increase due to water ALONE was only about 4%. They did get a temp reduction at the same time but I don't recall the particulars of that so I won't throw out numbers (it was a fair amount but not dramatic). 4% isn't the type of WEP I think is being discussed here, though. Normal max power is achieved when you reach the detonation limit*. Where the real power increase comes from with water injection is extra fuel/air can be fed in until you re-reach the detonation limit. So, now you're starting combustion at about the same cylinder inlet temp as non-water but your peak combustion temp is higher because of the additional fuel/air pumped in with the water.


* rpm + compression ratio raises temp a fixed amount.
a given fuel will detonate in a particular combustion chamber and rpm at a specific temperature.