Just to follow up on the EDM-800 DIF temp, did a couple tests. At idle, DIF read 18F. When converted, it incorrectly displayed 10C (should be -8C). Cylinder readings, in F, across the 6 for EGT/CHT were: 444/310, 463/322, 440/308, 474/329, 453/315, 468/325.
At around 1000 rpm, DIF read 22F, converted to 12C (should be -6C). Cylinders read: 531/234, 557/243, 528/233, 571/248, 542/238, 561/244.
Another note I had was that when rapidly shifting the throttle, the DIF temp seemed to settle much much faster than the EGT/CHT temps were moving, in that the DIF temp was stable while the EGT/CHT temps were still moving fairly rapidly more than approx. 10 deg. Not sure I'd expect the DIF stability when temps are moving so quickly.
Hope this helps!