Thoughts on it are all i have because i haven't had time to try either yet. I think you'll find a permanent undervolt is more efficient, if your phone can handle it, and you don't have to worry about Reflex's known problems (it's still not enabled by default because it causes reboots for some people). That said, i don't know what method Reflex uses to achieve its savings, only that it "is customized per device" in some way to take advantage of the capabilities of an individual bit of hardware. In essence, that's what you're doing (manually) by testing out different low voltages to find which one works stably on your phone. CPU power consumption goes with the square of the voltage so decreasing that is generally the most substantial way to directly decrease power consumption.

Blah blah blah...if/when i learn anything that lends more data and less thoughts on the matter, i'll share.