Thanks for the links. I'm going to have to look further into that and see what else is written about performance.
About one or many SCP connections, it looks like the fastest option may be to just use one scp connection.
I wish I knew more about how the connections work, then it would be easy to predict the fastest method. Specifically, if scp uses one tcp connection, then there is no real advantage of multiplexing. I've done some time trials but not enough to deduce what is going on behind the scenes. But it did look like SFTP was slower by a small amount in proportion to the amount transfered. You can do some time trials with your actual data set and see how much is actually used.
scp ./* server.example.org:/some/path/.
Then do the same for sftp in batch mode.
time scp ./* server.example.org:/some/path/.
For batch mode, you'll need to use a key.
time sftp -b upload.batch server.example.org:/some/path/