Not sure about the 800watt version but I did run the 200w heater and the controller was off 2 degrees. I will say it was very accurate at keeping the temp stable, you just had to add 2 degrees.
Also, the reason I don't run it on my tank now...when the power is switched off and then restored, the controller automatically defaults to 81, then add the 2 degree variant and thats too warm for my liking. Chose not to chance it.
They do make good products but this was my experiance with this particular heater.