You'll want to limit the ZFS ARC as well, to no more than 50% of the kmem_size_max.
It's the ARC that causes all the lockup issues, as it uses kernel memory. If you don't limit it, it tries to use up to 90-some-odd% of kmem_size_max, which means, if you have a lot of network traffic, disk traffic, zfs traffic, and so on all at once, you'll get the "kmem_map too small" issues. ZFS uses all the kmem, so there's nothing left for the rest of the kernel services to use.
vfs.zfs.arc_max is the tunable to set in /boot/loader.conf.
The fun comes when you start to play with all the ZFS properties, like compression (gzip-9 to save space, lzjb to save space without sacrifising CPU), sharenfs (configures the exports file automatically), and shareiscsi (can be used with zvols and the iscsi-target port).
Then you can start in on the fun that is "zfs send" and "zfs recv", which lets you atomically transfer snapshots between systems (either the entire snapshot is created, or nothing is saved). That's what I'm currently playing with, in an effort to create a remote, off-site replica of our backup server.
