Have you tried swapping the two antennas? It is unlikely, but possible that one of the two new antennas could be defective. You could also use the antennas from the C8 on the C8-Pro as they are identical. This probably will not solve the issue, but it is worth a shot.
Next question, how are you powering the C8-Pro hub? If via a PoE splitter, that could very well be the cause of the poor Z-Wave performance. See the following thread for more information.
