I have a 2004 legacy wagon and a 2006 WRX (previous owned a 2004 WRX).
Pinging with the turbo models were also major issues, especially the 2004. The ECU's seem to come from the factory tuned rather lean in an effort to obtain the ULEV status. I have had my 2004 legacy in to record my complain about pinging, but, the subaru mother ship claimed nothing was up. The ECU mappings are just stanky in an effort to lower emissions. If you see a 2004 dyno chart, you see similar activities - around 4000 rpm timing was being pulled because of knock detection (similar to the initial dyno charts in this, but not quite as drastic). A/F rations would run 14.5:1 and then drop quickly to 10:1 near the time knock was being detected. Knock was removed from a lot of ECU's via aftermarket remapping! YEAH! (I think a sarcastic smiley would be nice to have) So, running that close to 'perfect' combustion, adding something like a K&N filter which allows more air into the a/f ratio could be enough to trigger a lot more knocking because of being even more lean (as noted by the other dyno chart on this post - which was GREAT!)
So, higher octane or a really dirty air filter may help reduce knock. or maybe an intercooler... hmmm...
