Direct port injection is good for inlet valve cooling and precise metering. Most direct injection system uses a narrow angle jet pointed at the inlet valves. It is to maximising the cooling effect of water at those selected components.
As mentioned, without the use of some engine management mapped or a more intelligent ecu, the engine will not be able to take advantage of the effect of water. It will all depend on what the car is used for and depend on how far you want to go. Recently more aftermarket tuning tools are available but only a few willing to tune WI with it
WI on a standard road car should have post or pre IC injection, most modern management (un-flashed) will be able to learn new knock threshold at trim the timing to suit. Unfortunately WI injection seemed to be placed at the end of the list of performance modifications, by the time their turn has arrived, the engine managment has already been blocked out to do anything useful.
A standard road car straight out of the factory has certain amount of safeguard inbuilt such as EGT, knock retard and lambda sensor. Unfortunately some safeguard is being overdone - fuel dumping is the favourite because it is readily available and more effective than retarding ignition and dropping boost. If water inejction is avaiable as a standard fitment to combat those adverse conditions, it will be the most ideal tool to produce a "low emission" car with plenty of power.
Read this
link when SAAB published the results of WI on a eco engine. I have been closely following the project right from the start. It used port injection and produced great results. Pity it never got beyond the gates of the R&D site in Sweden.
For the entire bundle to work properly, the engine managment must be fully involved. At present only a few tuners are able to tune with aftermarklet ECUs with water injection effectively and the other 99% just repeating what the car makers were doing for many years - plus a few more pounds of boost with richer a/f ratio- not very encouraging and innovative.
One day the accountant in these big car makers will bow to the pressure of the environmental group and the performance enthusiasts to implementing water injection as standard - it just make good sense.
My answer to your question would be yes, if the orignal ECU has not been hacked and modified - for a standard production car.