Hi Rob.
I recently learned the WRX doesn't have an EGR solenoid so there's nothing I can check there. As for the knock sensor I haven't thrown any other codes than that P0304. My initial two P0400 and C0300 codes were incorrectly reported due to a software bug and there really was only the one P0304 code. However, thanks for the heads-up on the knock sensor and it's relation to these misfires.
Derrick