Kapton is a brand name for polyimide tape made by DuPont, much like Kleenex is for tissues and Xerox for copiers (once upon a time). Even within true DuPont Kapton polyimide tape there are multiple grades (
for examples) that range from 300-752 if not lower/higher. The limiting quality usually is the adhesives that are used, not the actual polyimide film although if you got cheap quality tape the upper range may be lower then what is expected. The tape rating also may not be for continuous use for long periods of time.
I wouldn't trust an IR thermometer unless it's a "professional" or scientific grade one where you can just for different emissivity ranges. Cheap IR thermometers are usually calibrated for an emissivity coefficient of around .95 which is near what a ideal object with peak emissivity would at. Aluminum has a coefficient of somewhere between .04-.3 depending on how polished and smooth it is. Brass is around .2 or less if it's polished. So presuming that your heater block is brass or aluminum since almost all are, if you're reading what you think is the correct temperature, you may be reading a much higher temperature since it has such a lower emissivity coefficient.
Your thermistor may also be defective, use a different thermistor table then what your printer is configured for, or there is poor thermal contact between the block and the thermistor. I know the first thermistor I had was nearly 14 degrees different then what the table suggested it should have been until I generated my own table specifically for that thermistor. I don't know if the table was oudated, my thermistor was defective, or what.
