20 amp install dilemma:15 feet PC OK?

If you use and old work box you will not have to do any repair work. check them out at any hardware store,

If the extension cord is heavy duty, and you are ok with the safety aspects of permanently having the exposed power cord, you ought to be aok.

I had the same issue that you have. I ran a PC straight from the fuse box in the basement, into a cryo'd outlet behind the gear. The length was approximately 45 feet. I used cryo'd romex from Chris VenHeus here: http://venhaus1.com/VH_Audio_Test.html
I bought 50' of techflex expandable sleeving from Parts Express.
It looks fine and works great...it even sounds good!

About a year ago I did a similar thing but instead of terminating into a receptacle, I terminated with a 20a IEC and plugged straight into the back of the power amp from the circuit board. You can do either, both work well.

Rooze