I believe that info about damaging your cat is fairly dated (note leaded fuel reference) and mostly applied to older cars with mechanical fuel pumps and carburetors.

Here's a EGT probe/pyrometer, which seems to be pretty good. Imo, this combined with some time with a local smog guy would be the best bet for figuring out the relationship between how cool you get pre-cat and how well the cat operates/how high HC emissions when the engine is started back up, so you can figure exactly what temperatures are best for EOCs/MPG/HCs.