how hot how far how can I trust it
Distance from the face of the piston/combustion chamber is the issue. on our wooped sleds we found 4" from the face of the piston worked, that's
pretty close to the start of the pipe.
So how hot ? where you place the pyrometer and how rich or lean your carb is delivering gas can make a big difference in temp reading. In the wrong situation a 25% rich condition in the combustion chamber will send gas going out the pipe and still burning, often 6" or more down the pipe you will have a hot condition 1350 degrees, but a cold over rich condition in the combustion chamber.
Too lean in the combustion chamber, no unburned gas escapes down the pipe and 6" out it reads cold 850 degrees, don't be fooled, going leaner will chunk da piston.
So the Pyro's can help you and give another opinion, but learn to recognize deto, lots of visual looks at piston top............tough on bikes, how to read deto on plugs.
In the end, Lectron and EFI will show different temp/plug/burn color than the leaky MIC/Keihin
When you know your jetting is near spot on, the pyros are good to tell that up one or down one jet, tells you not to chop the throttle at the top of a long steep climb, lots of interesting information, just don't hit a tree or off a drop while reading the temp gauge.
Used to be lots of good write ups on pyros on two stroke sleds, and good info from the dyno guys, and it mostly came down to tuning and avoiding deto looking for good power.