Stock kernel uses Interactive governor.
As to undervolting and safe voltages it will vary from phone to phone. So the best way is to read, then read some more, and then read. Read some more then you'll have a good idea
But seriously, you'll have to trial and error it yourself after getting an idea of what most people are running safely. Drop them 25mV at a time until you get random reboots. Then raise them back up 25mV and test for 3 days
before "setting on boot".
For me, yes there are very noticeable differences between kernels in both response and longevity. Some people who've been around the Android OS for a while are more receptive to these changes.