Well, even though the extension lives on the local hard drive, it could still make loading it faster: At least with HDDs, people made experiments gzip'ing the Linux kernel in /boot (or parts thereof) and using the CPU to unzip it on load, which was faster than loading the uncompressed version (no source at the moment, sorry).
So since minified code shouldn’t even need uncompressing, I could imagine it to speed up the time it takes Opera to load the extension – if you always start a new browser if you want to go to a website, that might even make sense.
Comments
Why is minification important in this case? How will it effect your execution of the extension?
Well, even though the extension lives on the local hard drive, it could still make loading it faster: At least with HDDs, people made experiments gzip'ing the Linux kernel in /boot (or parts thereof) and using the CPU to unzip it on load, which was faster than loading the uncompressed version (no source at the moment, sorry).
So since minified code shouldn’t even need uncompressing, I could imagine it to speed up the time it takes Opera to load the extension – if you always start a new browser if you want to go to a website, that might even make sense.