Yes thermal information does exist for the HMC358MS8G, unfortunately it's not currently shown on the datasheet.
Simulated results using the hottest devices and their physical placement and geometries estimates a typical junction temperature of 122.8°C at an 85°C baseplate temperature. This is based on a nominal bias condition of 3.0Vdc @ 100mA and yields a thermal resistance of 128.7°C/W. Per the foundry, the maximum junction temperature that will result in an MTTF of 1 million hours of life at a current density of 50KA/cm^2 for the GaAs InGaP HBT process is 135°C. HMC products typically operate at a significantly lower current density (20KA/cm^2) so this analysis should be viewed as very conservative.