Sub zero does one thing....conversion of RA. There is so little RA in those steels you won't notice any change in performance. Now, if you're aus temp is too high, and thus getting a lower HRC post quench, then you likely have too much RA. Back off on the temp, the HRC goes up and will max out at some temperature (usually in the 1475F range for hyper eutectoids, and 1500F for eutectoids), and the RA will be extremely low. You may be able to convert more of it with the sub zero, but your HRC will not go up, thus telling you that VERY VERY little was converted.
Yes, sub zero or cryo (LN2) is best done immediately after quench to room temp, and prior to the first temper.
There will be extremely little to NO benefit using sub zero with those steels. However, LN2 is a different story. Not only does it convert the RA, but allows for the precip of very very fine carbides upon tempering. Topic for another thread.
