If someone could actually prove it accomplishes something I'd be willing to build up a intercooled system but I'd rather build my own system than buy it.
Spearco makes very nice intercoolers but their prices are a little out of hand in this situation, I'd either use a Starion or DSM intercooler in this case. I'd probably go with DSM intercooler as I have access to cheap ones and they are a very efficient intercooler.
Injectors can be extrude honed if the money is right, but before I'd consider sinking much into this car I'd probably get something else much more modifiable (Dodge Ram w/Cummins?
).
All I can figure is Subaru's marketing team must have forced their engineers to have a top mount intercooler. It just doesn't make sense why someone would do that, heat rises and they put it right above the motor. One of the first mods for a WRX is to rid of the top mount intercooler.
The current tranny may put the power down but will they last? The 603 will put put out a good deal more power than the 617 but most of their tranny's seem to fail every 100-200k miles as well.
Yes the intercooler would create a little extra turbo lag as it is that much more volume it'd have to pressurize before it could reach the engine. I'd be much more concerned with the effect that driving in the winter with a intercooler than this issue though.