Extensive simulation studies have been carried out for the first time on the feasibility of THz power generation from an InP based Double Drift Transit Time diode operating at elevated junction temperature (2500C). The effects of mobile-space-charge on the THz-frequency performance as well as on the parasitic series resistance (RS) of the device are also investigated by a generalized simulation scheme. The study reveals that at the optimized bias current density of 3.2x108 Am-2, the device is capable of delivering output power density of 3 x109 Wm-2 with an efficiency of 7%. With the increasing bias current density the space charge effects are found to become prominent and this cause serious degradation of THz performances of the device as far as output power density, efficiency and negative resistivity are concerned. It is observed that at a high bias current density of 7x108 Am-2, space charge increases the value of RS significantly (~38%). These optimized simulation data may be suitably used for fabrication of InP –IMPATT device at 0.3 THz region for application in high-power THz module.