I enabled my iimb by hand. Seems to work fine on my 6100.
FWIW, the current documentation indicates that the default value of kern.crypto.iimb.enable_aescbc is 1 (enabled), although it has a warning that iimb can be slower than qat for cbc. I don't use cbc, so it doesn't matter in my case.
I think the documentation is incorrect or outdated (at least for the 6100), as the code in /etc/inc/config.console.inc explicitly sets kern.crypto.iimb.enable_aescbc to 0.
FWIW, there is also an interesting note on the qat/iimb trade-off earlier here. YMMV