We present an accurate extension of the coupled spin and charge transport model used to compute the spin-transfer torques in nanovalves to enable the anlysis of ultra-scaled magnetic tunnel junctions. The charge current behavior is reproduced with low conductivity locally dependent on the angle between the magnetization vectors, while for the spin current proper boundary conditions at the tunnel barrier interfaces are introduced. We demonstrate that the experimentally measured voltage and angle dependencies of the torques acting on the free layer are accurately reproduced, and that using our extended approach is key to accurately capture the interplay of the Slonczewski and Zhang- Li torque contributions acting on a textured magnetization. The described approach is successfully employed for switching simulation of recently proposed ultra-scaled MRAM cells.