Power cord - 5 ft vs 10 ft ?

Theoretically, since all power cords have a resistance per foot, the ideal length would be as short as practical. If by 'performance' you mean as low of a voltage drop to the amp as possible. 

@jasonbourne52 I can move my amp to be one inch from the outlet. Any idea where I can get a 1” power cord? Or should I just hardwire the amp to wall?

@jumia you’re overthinking it. Here’s why….a 1m AudioQuest Hurricane will be way better than a 3m AudioQuest Thunder for nearly the same price. Get where I’m going with it? Quality over quantity. Unless money is no object. 

For power cables my understanding and experience suggests that shorter is not better. Depending on cable, at least for the ones i use, 1,5m-2m is perfect.

Short as comfortable.  Using shielded will ensure you have the best noise blocking (both in and out). 

There is something to be said about using a longer shielded cable.  A little R and C can help reduce noise, but by a tiny amount. 

Dear @jumia, If you’ve heard "many things" about the length of power cables, you may be reading too many audio boards! 😉 There really isn’t that much to say!

If this is an effect at all, it’s vanishingly small compared to things such as speaker and component selection, speaker and listener positioning, acoustics, subwoofer positioning and integration, choice of listening chair, and so on. And the more subtle of those are absolutely swamped by variations among recordings.

As a rule, one could do far worse than "long enough to reach the wall outlet."