Probably luck here.. I have an 803T in a project box/NO heatsink/ AiXiZ module. It has over 5 hours of runtime over a 2 month or so time period, with 1-2 minute duty cycles. I have never had the module's temp(right at the diode) go above 90 degrees F. This also means ALOT of on-off cycles...
Of course it all seems to depend on the paricular diode.....