Water vapor injection works and can decrease combustion temperatures and octane requirement as well as increasing power. A well-designed and integrated system can pretty much do as they claim. But, you have no way of assessing what will happen until you drive it, and if it is poorly adjusted or designed, it WILL damage your engine. At the least, you could get hydrostatic lock, which would be catastrophic.
I would find testimonials from the exact kit installation on another similar car before risking your investment.