There was some major changes to the C8 audio engine compared to previous versions, but those changes were more specific to audio ASIO/CPU performance. I would have my doubts if there were a difference in something that was obvious such as phase, freq response and/or dynamic response as that would not be expected.
But I do seem to note that C8 being a little snappier, tighter, smoother in it's use compared to previous versions. Likely due to it being more mature and having better optimized code that would also be part of any improved performance that it provides.
I have not compared audio quality from C8 to previous versions but it is possible that something related to an improvement to their VST implementation and/or the audio engine overhaul has done or has changed their mixing/summing math that you would not hear on individual raw tracks but could hear with tracks that are mixed together.