I think the main bonus with water meth injection was the theory behind it.
Your adding a substance which each molecule is made of 2 oxygen and 1 hydrogen..
Oxygen is what gives the burn.
In 1 cubic meter your getting only 27% oxygen, so not very efficient.

And the transfer of energy. The water absorbs the heat and turns into vapour..

There are lots of bonuses.. But there are downfalls. Water weight is a main.
The setup can be a pain.

If I was looking for that bit extra easy I'd go it..