If you have, then sitecopy might not be the best program for you to use. Have a look at rsync first. rsync can handle changes to existing files much more efficiently than sitecopy, by only transmitting the differences: sitecopy will always completely re-upload changed files. On the other hand; sitecopy can handle moving and renaming files more efficiently than rsync; it can simply move or rename the file on the server, whereas rsync will completely re-upload the file when it is moved or renamed.
If so, then bingo! You've come to the right place.
Well, congratulations. It's your lucky day.
Short answer: mirror is designed precisely for mirroring large FTP archives. sitecopy is designed precisely for mirroring local web sites.
Long answer: You can use mirror instead of sitecopy - the basic functionality is duplicated. But, sitecopy does not go to the FTP server and see what's there every time - this is the fundamental difference between sitecopy and mirror. This saves bandwidth, and startup time, especially so for slow servers holding large sites. Also, sitecopy has some features missing in mirror (though the reverse is also true), such as remote file move and interactive confirmations.