Are manufacturer AC cables good enough?

djones.... Clueless clump of cells, I absolutely love that ! Certainly wouldn’t Seek your opinion of cable risers,cards,pastes,speaker cables, pads, or any other Magic Band-Aid product out there. That was funny !

Cheers 

appropriate for the job. After market can offer way better performance at a cost.

The power cable included with your amp if perfectly adequate for your amp to work well.  High end amp manufacturers accept the market forces that dictate having detachable power cords.  From an engineering and cost perspective it is better to have a non-detachable power cord.  One less interface for those electrons to deal with and the connectors are usually the most expensive part of a power cord.  Additionally, many amp manufacturers simply don't buy into the power cord thing.  The might not say it publicly, but they think it's a non-issue.

As long as you're willing to settle for, "good enough".............