With regards to injecting in front of intercooler - the water and charger temperature eventually will reach an equilibrium vapor/liquid/temperature relationship. The difference between equilibrium between the entrance to the intercooler and the valves and the equilibrium from the throttle body to valves would not vary significantly on a WRX. This is due to the long runners from the throttle body to the valves on a flat 4. Equilibrium is reached quickly - despite the very long distance from the entrance to the intercooler I do not think there would be any more evaporation prior to the valves than there would be from the throttlebody - IMO.
There likely will be no real benefit or detriment of putting a single jet at one location rather than another - with the exception of installation. Given the layout of the WRX and the best location for the installation the entrance to a FMIC is logistically a good location. If the throttle body is logistically better then no real reason to not go there.
You may want to see the experience that was had by a tuner in England while running on a dyno with one jet before the intercooler and one before the throttle body -
http://www.turboice.net/forum/viewtopic.php?t=12