My Garmin Forerunner 305 was predicting calories much too high and never utilized input from heart rate strap. The Edge 800 utilizes an advanced correlation including heart rate data.

My Edge 800 is determining the calories to be about 60% of the Forerunner. If the Edge 800 is predicting 30 cal/mi or greater, it is providing realistic values. Those having serious issues with calorie predictions are recording less than 10% of realistic values. Many of those with ridiculously low calorie predictions have had success with a hard reset. The Edge 800 also has an option to enter fitness level. This setting may take a little trial and error for tweaking the calorie predictions. I am currently using fitness level 7.

I forgot to mention the Edge 800 obtains a satellite fix very rapidly. The 305 was slow to obtain a fix.