in terms of boiler heat/temperature, i would reasonably assume the boiler mfg. constructed a boiler that meets asme code and that is performance/operation is satisfactory. regarding glycol, yes there is a temperature limitation for glycol/water solution. the main concern is the film temperature of the glycol/water solution. if the film temperature is above a certain temperature, then the glycol/water mixture may turn into carbon. i suggest that you contact boiler mfg and have them provide film temperatures of various parts of the boiler - if this is possible.
you may also want to consider increasing flow rate through system - if possible.
-pmover