To add to this very belatedly, I've got some more empiricle answers to the above questions having done the install of one of these (on a 05 WRX) and done the before, after then post tune dyno's while the parts were being installed. And here's the low-down.
1. What it does -> Increases airflow into the turbo. Specifically, it reduces the effort required for the turbo to suck air in which means it can suck more air, faster and easier..... so the turbo spools faster.
Ultimately, you won't really get any or much more power but you WILL get noticable throttle response improvement.
Here's a graph of Torque changes on a 2l WRX with a stock TD04. You'll see the torque flattens as the inlet track is openned up:

This is Boost change as the pipes are replaced, again, you'll see the extra airflow/restriction removal lets boost peak earlier and earlier:

And the power changes according to this graph (note the peak power drop when the intake pipe is installed). You'll

2. Do you need a retune? -> ABSOLUTELY! The >MY01 ECU's (when they went 32bit [I think]) in Subaru's are horriffically sensitive to significant airflow changes. These kinds of parts changes will result in massively rich running and plenty of spare fuel in the exhaust (crackle).
You can see the massively detrimental affect of openning up the inlet track. AFR goes into protective mode and fuel economy goes out the window. That's why you need a retune! And also why you get CEL's.

And here's what it looks like after you retune it (power and torque) NB: car was tuned once before these parts were fitted:

And here you see less boost <3750rpm but equivalent or more power at the same RPM's. That's pretty much due to the more open intake letting the air get in easier.:
