I pretty much agree with all the above. It is very hard to give a fixed formula for how much water is right for your needs.
It is always an experimental process. As mentioned above by John start out with the 10%-20% water to fuel range and see what the engine likes.
WI should lower your engine temps "slightly" but not a lot.
If you engine runs hot for everyone it is probably a characteristic of the engine and car configuration. Oil coolers would help but there are also some other things you can do.
Have you tried any of the cooling additives like Redline's "water wetter"?
What water antifreeze (ethylene glycol) are you running?
Here in the U.S. most folks run a 50/50 mix of antifreeze and water to provide freeze protection in the winter. A 25% antifreeze and 75% water mix gives noticably better cooling especially if water wetter is added to the mix as it improves heat transfer from the engine to the water.
Check your radiator to be sure you don't have a mineral build up in the radiator. If you do, you will need to clean the cooling system to flush out the mineral scale and give good cooling.
What thermostats are available for your car?
Many times you can help the situation by running a thermostat that is 10 deg F cooler than the factory setting.
What weight oil are you using? Sometimes excessively thick oil can actually make the engine run hotter.
They make special coatings that help with heat transfer that you can get the oil pan coated with and that will work much like an oil cooler to lower engine heat load.
http://www.techlinecoatings.com/arti...an_Article.htm
It would increase your water usage but a water spray on the intercooler and or the radiator might also help your engine heat issue, but that is not really water injection.
Water injection usually requires a "small" increase in engine ignition timing but this varies with the engine and the fuel air ratio so must be worked out by the tuner.
Hope that helps.
Larry