Yes, there is both a practical and a safety issue with the maximum water temp in a radiant floor. The floor temp shouldn't end up much warmer than your desired air temperature, and once the temp in the slab is stabilized, the feed water normally wouldn't need to be very hot, maybe in the 100-120 degree range, depending on how deep they are and how close together, and, how well the slab is insulated underneath. How well the whole room is insulated will also dicate how much heat you need to add. Somebody that does this all the time should chime in with some max values, but they're likely much less than the straight boiler temps.
Have you done a heat load analysis? Knowing the amount of heat needed, and the available length of the pipe and the flow rate would give some idea of the heat transfer capability, then setting the temp becomes easier.