SQL Server Log Entry Gathering?
However, what I'm really curious about is what else might get flushed along with that.


Any log lower than the LSN currently requesting to be flushed will be flushed as well. Log records are put into the log buffer in order and all transactions are serialized to the log buffer for that database. Note that individual log records aren't flushed, they are put into log blocks and log blocks are flushed.


  Does anyone know if such an optimization has been made in SQL Server?


It's called group commit, and it stalls the normal commit for a few ms if it can group more together for better overall performance (roughly-ish speaking). I believe it was added in 2008R2.