Back in the day when I was a research/design engineer we did quite a lot of work on cryogenics, and 300 Below are really, really good people to work with. We found the most improvement in strength/ductility/microstructure when dealing with certain alloy and tool steels in the 45-50 RC range. Well above what a bbl would be.
That said, it can't hurt to try. Never saw cryogenic treatment do any harm. Just depends on whether or not you want to spend the $ to find out.
alan