Yeah storage groups are the way to go. You could give some space to tv recordings and mythvideo if you wanted to do so. I'm happy that LVM is a thing of the past for mythtv.
My only tip would be to make directories under the mount point. For example lets say your new drive partition is mounted on /mnt/storage2000, make a directory there "tv" owned by user mythtv. Then add to the storage groups for recordings using mythtv-setup the path /mnt/storage2000/tv. If you do this it will allow you to add another directory to then add to mythvideo if you want.
I am using storage groups for mythvideo too. The only problem is if you do use storage groups for mythvideo, then you must use the internal player. And the internal player has issues with playing DTS audio. The internal player for mythvideo/storage groups requirement should be fixed in 0.23. In the mean time you can stick with NFS and multiple directories for mythvideo if you cannot live with the Internal player. I re-encoded my videos that were DTS to AC3 to get around the problem.
The upside to using storage groups for mythvideo is the file browser in the front end. When you use NFS and multiple directories you get a "folder" for each directory. That works well until you go to add a new drive. For example say on your first drive you have a path /mnt/drive/movies/adventure because you like sorting out your video library by genre. Well lets say your drive is full and you add a second drive. You add another NFS path /mnt/drive2/movies/adventure. In mythvideo browser you will see two folders both labeled "adventure". Which is dumb if you are sorting by genre!
However if you use storage groups, mythvideo is smart enough to put both sets of movies into one folder in the browser. Much nicer and friendly to adding new drives and video cleanly.
Perhaps this is a bug but it's the way it behaves in 0.22.