There are a number of intakes out there that do require a tune for them to work properly (and safely). The reason that an intake would require a tune would be because the intake creates turbulent’s before the Mass Air-Flow (MAF) sensor, or due to its design, it causes the highest velocity air (usually in the center of the air flow) to miss the MAF sensor. Also, some intakes do not retain the original internal diameter of the factory intake plumbing. Any of these conditions can skew the MAF’s calculation of the amount of air coming in, and will require a recalibration.
Different intakes can create these conditions to various degrees, but in the case of an extreme discrepancy in the MAF signal, this can actually throw a Check Engine Light for the MAF sensor reading out of range. If this happens, it is recommended that you remove the intake as soon as possible in order to take your car to have it tuned with the intake.
Fortunately, this does not apply to the SPT Intake because it was designed by Subaru to work correctly, and not need any custom calibration.
Probably the strongest evidence of this is the fact that the SPT intake is one of only two performance parts (the other being the SPT Cat-Back Exhaust) that you can install on your Subaru that will have no affect what so ever on your warranty. It is doubtful that Subaru would offer this coverage if there was any chance of the intake causing an issue as described above.
Beyond that, we have done our own testing of the SPT intake, and you can read about our results in our Project Car Blog. We did this test almost two years ago, and the SPT intake is still on our car, working flawlessly.
One other question that comes in regards to the Legacy GT SPT Intake specifically, is how it can work correctly with the MAF sensor placed so close to a bend?
If you look at the stock air box of the Legacy GT, it is entirely different than the Impreza air box, and if you look closely, you will notice that the placement of the MAF sensor is just to the side of the air box, which would require the air to make nearly a 90 degree bend. This requires a different design for an intake, and is most likely the reason for the difference in the design.