You need to remove the latch mechanism for ideal contact. I did this sometime ago with a QX6700 (I may have been the first actually...) and I found I got "passable" temps with the hold-down still intact, but once I removed it (possible to do just by bending it a little so it can be put back on) contact was much better. The arm can be left on depending on the heatsink you're using, but the hold down has to go.