Too low - water-cooled reactors are designed to bring the water in the secondary cooling system to just above the boiling point, because anything beyond that is wasted energy (the temperature of the steam doesn't really matter as far as driving turbines). So retrofitting a molten-salt system onto these reactors isn't very efficient and you would ideally want a new reactor design intended for that from the beginning.
> (the temperature of the steam doesn't really matter as far as driving turbines)
This goes against everything I seem to remember from thermodynamics - superheated steam is exactly what is used for driving turbines, with saturated steam as the cold side outlet of the turbine. It's what makes district heating useful - there is still heat energy left in the saturated steam, too low grade for driving turbines, but still useful for heating apartments etc
"We need this value to be more specific numerical form to utilize in design of high temperature and high pressure steam boilers and turbines working at 300 bars."
It seems like a serious materials challenge problem to come up with a molten salt that has the right nuclear characteristics (neutron moderating, etc...) and is not horribly corrosive/flammable/etc... Especially if your usage model is to let the salts heat up to outrageously high temperatures during the day and then draw them down over night.
Are the temperatures too high or too low?