Several years ago I did a lot of wire (10/2) and outlet comparison testings (check out my older reviews and comments).
I concluded the following to answer your questions:
1) It's a personal matter of diminishing marginal returns.
2) I did not notice a difference in sound when my outlets were located close to each other or separated by six or eight feet or so, however there is nothing wrong with the idea of separation of outlets.
3) I did notice a difference in sound dynamics when using different high-end outlets and it's okay to mix and match different outlets and AC wire to create the desired sound you want to achieve.
Feel free to email me if you have specific questions.
I concluded the following to answer your questions:
1) It's a personal matter of diminishing marginal returns.
2) I did not notice a difference in sound when my outlets were located close to each other or separated by six or eight feet or so, however there is nothing wrong with the idea of separation of outlets.
3) I did notice a difference in sound dynamics when using different high-end outlets and it's okay to mix and match different outlets and AC wire to create the desired sound you want to achieve.
Feel free to email me if you have specific questions.