umm, if it's branded "pyrex" it's supposed to be borosilicate glass, not soda lime -- at least that is the limitation of the US trademark. corning's web site still says that it is borosilicate
to address the OP's question, i agree with comments above that coating the heater with rubber, while toughening it relative to mechanical and thermal shock, would substantially reduce the heat transfer. to get the same amount of heat through the coated walls, the insides would have to get at least twice as hot (estimating doubling the thermal resistance, which i think is very conservative); the innards of the heater are never going to be able to stand that and something catastrophic should happen pretty soon after it's plugged in.
