After finding a better document on p7zip (/usr/share/doc/p7zip/examples), I've found the command options
7za a -t7z -mx=9 -ms=on -mmt=on
do a much better job of compressing, bringing the total in line with the aforementioned torrent. From 2 DVDs to a quarter, is good stuff I think. There is a downside, though: ZSNES and snes9x don't support this archive format natively. But they didnt support the grouping format with zip either. So no huge loss I guess.