Two new surprising properties of stimulated parametric scattering of cavity polaritons in GaAs based MCs have been observed and discussed. First, the expected increase in the threshold power for the stimulated scattering P thr with temperature T appears only in MCs with shallow LP branch. With deepening of the LP branch the dependence of Pthr (T) changes unexpectedly to opposite one. Second, at low T Pthr displays a drastic decrease already at very weak additional excitation above GaAs band gap generating hot excitons and free carriers. Our observations indicate that in addition to the FWM formalism describing only four waves participating in the parametric process, and bistability of the amplitude of pumped macrooccupied mode, the full physical picture including other relaxation channels should be taken into account to reproduce the observed non-linear behavior.