You best approach is to get with the manufacturer of the equipment to ascertain why the temperature limitations.
The following is just a simple outline of your problem.
In generality will depend on what function the CS is playing like if is just a skin with virtually no load yes you can go higher in small areas than your 300C probably up to 700C-800C before bad scaling takes place. These very hot areas on the skin of a furnace or heater are called "Stingers" and are normally caused by failure of the insulation at that point. Your numbers are probably correct for any structural component with the 170C representing normal operating temperature and the 300C for excursions. Some areas may be limited to temperatures less than 170C for protection of personnel and instruments. Stop,think, and ask before any action is taken. I worked with and around furnaces, heaters, and boilers for my entire career and was always getting surprised with the resulting head scratching.
Normally if you have a Stinger there are several approaches to mitigating it starting with an outage to correct the problem. This is especially true if the Stinger is on the roof of the furnace. Some Stingers on sidewalls can be repaired from the exterior by injecting a pumpable insulation material into the site. Depending on the function and operating conditions of your equipment it maybe be possible to repair large areas by Ceramic Welding. Until some form of corrective action is taken you may have to resort to the water hose or spray nozzle. Just remember that Stingers don't get smaller or cooler.
Again remember that the above is an outline and should be used only with a thorough understanding of how your equipment operates.
If you have ny question comeback with it and hopefully you will get some guidence, if nothing but call the manufacturer or a refractory company.
