All manufactures do the best with the technology available to them at the time, so while in an ideal world everything would be musically perfect, in the real world it may not be as accurate as you think. (Time stretch takes a lot of processing power)
A good example is the fact that most manufactures are now moving towards or are already using sound modelling for their sounds rather then sampling, (Giving a much more real sound) which requires more processing power then is available in hardware based boards, however Yamaha developed a Hybrid of the 2 called Super Articulation which while not accurate still gives a fantastic sound. (For Yamaha owners who think SA is perfect, ask yourself why Yamaha are bringing out SA 2 in the soon to be released Tyros 2 replacement)
Remember, due to the time taken to develop a new hardware board (2 – 3yrs) the technology is quite old by the time it is released. (As an example the Audya thread has mentioned a P3 CPU which is positively antique, and P4 has almost disappeared)

Bill