Smoke, I am not saying they are crap or that they are of no use, I just stated keep in mind the temp is slightly different then what the actual glow plug temp is. Plus each engine runs differently. I have had big blocks that wanted to run at 270 only and others which ran like a champ at 225, same manufacturer and same model.
Tandum, its actually much more predictable than you think. The glow plug is shielded from all sides and that little area in there is going to hold heat more than the outside of the head where there actually is airflow. The temps do not change that rapidly in there at all. All I used last year was IR and I would warm my 1/8 scale on a starter box, RB S7, throw it on the track when it hit 210 to 220 and after a 5 minute heat at WOT, it would come of +- 10 degrees to what it went on the track at.