Huntindog wrote:
Weld a big nut to to it. While it is still hot, get a wrench on it. Heat alone usually works pretty good at breaking things loose, but since it is already rounded, the welding of the nut will fix both problems.
There's your best answer.
If you rounded off the head while using a 6 point socket, no easy-out is going to break it loose and I rather doubt a pipe extractor will either. Although a pipe extractor may work if you first heat the plug and cool it rapidly. Sometimes the heating and contraction is enough to break the threads loose.
Just leaving it in and ignoring it is going to cause you problems, most likely at the most inconvenient time.