My wife's 97 Outback Limited and my RS have the same setup with the exception that I put a 3" mandrel bent Al tube between the MAF and Torque box in the RS. The Outback has that harmonic hum at idle and the RS dosen't. The air is cooler in the fender driving around so that should help some.
Temp info here is for the RS but it should be similar with a Legacy I would think.
Personally I feel that the Snorkus will be somewhat restrictive. Also if you look at the above thermal info it's grabbing hot air from the motor bay even when the car is in motion: