The specialized aviation adapter cables are fantastically overpriced (there must be a law somewhere that companies are required to rip-off aviation consumers). Assuming you have an open headset port on your intercom, save yourself a few bucks and just buy a 1/8 to 1/4 TRS cable. You can get it on Amazon for less than $7.
Unfortunately, if you have the Hero 3, you're still stuck getting a USB audio input adapter (you would need this for any type of external audio input anyway). But even then you're only out 17 bucks, versus spending $44 (!) for a specialized cable.
Regarding the high signal output, this doesn't appear to be a problem with the GoPros; I've successfully recorded a bunch of flights this way with no issue. Besides, as far as I know, none of the specialized cables provide any type of impedance matching either.