Updating iTunes Match metadata/tags?
I had a similar issue, it seems as once the song is uploaded to iCloud via iTunes match it becomes the "Master" copy.  Install iTunes 10.5.1 or later on another PC and connect it to iTunes Match but DO NOT download anything!  Then you can delete songs from iCloud, don't worry this won't delete them from your main iTunes library. Once deleted go back to your main iTunes machine and change the metadata, then choose add to iCloud for the song. The new metadata will now be ULd to iCloud.