I too noticed a big difference in temps between two E8400 chips. The bad Q817 one was cool but didn't like to overclock, did 1.3xvcore with load temps around 55c. The better overclocking Q815 gets to 60C on 1.18vcore.
I've checked and remounted my Xigmatec 1284 a couple times now, still the same difference.
Another thing I noticed was a big difference in vDroop, though Load line calibration on the P5Q nullifies it with both chips. The vDroop on the Q815 is alot higher.