I didn't actually solve it or do anything.
I just upgraded to RELENG_8.
Now it's behaving more like FreeBSD should.
I can do sequential reads/writes and still
use kbd/mouse/X11/buildworld and so on.
Yes, the cpu is still completely underwater,
and allowing for that, it seems interactivity
is nearly back to normal. ZFS is taking up
a power of 10 less cpu than before.
Geli is at it's rightful place on top of the
cpu/time list. And where copying GiB sized
files would sink the system requiring the
reset button, it just keeps on copying.
If anyone's having issues with similar setup/loads,
all I can say is upgrade asap.