If a file within an archive is unneeded it gets moved to the backup folder and since 7z does not support a direct copy of compressed data, you always have a decompress + compress step, so it doesn't really matter if zip or 7z is used. Actually zip would even be faster since it would support direct copy of compressed data (but currently I don't know if this is used when backing up data...I somehow doubt it).
Generally for backup zip is used. You can't change it.
"waste, because files will often be re-compressed with 7-Zip again"
erm...why? If cmpro moves files to the backup they usually stay there since they are unneeded.
"Too much time"....well, usually (unless you scan a completely wrong path), you only have some unneeded files which get moved to backup...so the time argument is not really that important in my opinion....and as I said, no matter if you use 7z or zip, the files get decompressed and recompressed anyhow just for backup.
Maybe I change this for the new scanner....