1. Without any idea of how your geometry looks and what your physics is, it is not possible to comment on the values you've obtained. Regarding your boundary conditions question - if you know the mass flow rate in your problem, please use mass flow rate inlet and pressure outlet to solve your model. Regarding your comment on obtaining supersonic velocity, how good was your convergence? Are you using a density or pressure-based solver for this? Is this a steady-state model? It is important to have some understanding of how you are modeling your problem to be able to comment on your solution.
2. If you are aware of your mass flow rate, then I'd recommend you use mass flow inlet BC in your model. Mass flow inlet and pressure outlet is a good combination for a robust convergence.