Coming in halfway along this discussion, sorry. Everything I know about water injection is pretty much what's on Aquamist's website, and it's very good. The science seems sound and all your questons are in there (or at least were when I last read it!). You need to put the injector as immediately downstream of the intercooler as possible to allow good vapourisation. You're not putting a mist of water into the intake, you're looking for it to vapourise, and that takes time - the longer the better. This takes energy out of the combustion process by robbing heat and therefore expansion, but mostly this allows you to burn more fuel more efficiently, so the benefits tend to outweigh the costs when you're running a lot of boost.
Ref fans on intercoolers.... it's an air-to-air heat exchanger and it makes sense to me that you want to flow a similar amount of air over the cooler as you're flowing through it, if you want a decent heat transfer to take place. At high boost, say a bar, on a Z7U (if my memory serves and without recalculating it) at 5000rpm, the IC is flowing 166litres of air per second.. I'm not sure even thumping great big rad fans flow anything close to that, so you're left with relying on the thermal mass of the intercooler and the fan's ability to cool it back down after a power run... which leads neatly into when it DOES work.... which is when playing with chargecoolers like Dave's, because they have a heowge thermal mass compared to an intercooler. You're not doing a direct heat exchange, you're tranferring wasted heat in the intake to heat up a load of water. Unless you're driving flat out for extended periods (minutes) then you'll be able to lose that excess heat through a comparatively small rad over a longer period than the 1/4 mile run
If you are running flat out for minutes on end, you're probably going quite fast and airflow over the pre-rad should be at least marginally healthy, depending on placement of course.