I have just signed up with iTunes match (on friday) so i have been going through all my songs, like you deleting and downloading the higher bit version where I can.
Also have been trying to get songs matched (instead of uploaded), I had a pretty good success rate (only 10% were not matched). Some of the ones are strange, one or two songs off of an album would be uploaded while all others are matched. These are albums in iTunes so i am not sure why some songs are uploaded and not matched.