B- depends on how resolving your system is. The answer to your question is exactly what you would think it would be. Again, all systems are different. I now have p-4s from the wall to the P-10s and from the P-10s to the amps. Better than p-3s and much better than stock cord, which is still better than most other stock cords I've heard. P-4 is audibly quieter in my system. P-10 is audibly quieter than P-5 in my system.
Stevepo - curious to hear how you like the HD8's. What are they replacing and what are your speakers?