I actually had bigger gains undervolting the cache vs the core, although I have undervolts for both.
Same rules apply as overclocking, do as much testing as you can to gauge stability and remember its never an 100% assurance as ultimately you are running out of spec. There has also been a couple of reports (one from ir-cow and another from buildzoid) that undervolting can cause the CPU to slow down, however I think in both cases they were undervolting whilst also overclocking making it a more aggressive configuration, both using asus boards I think.
Easiest to test with with XTU as dont need reboots to make each adjustment.
One of the reasons CPU's might ship overvolted or bios's might pre configure overvolted that isnt related to binning margin is that adaptive voltage will overshoot when it moves up and down, so there is an additional margin to keep the CPU stable when it has downward overshoots. Motherboard quality according to buildzoid can impact this phenomenon with the better boards allowing a more aggressive undervolt. Fixed voltages can be a great way to overcome this issue at the expense of higher voltages when idle.