Undervolting is the new overclocking. Classic overclocking is made obsolete by the lack of overclocking headroom in new chips. Today's chips by both manufacturers make a good effort using turbo/boost speeds efficiently, pushing the cores to the max possible frequency when in use and back into sleep state when idle. Undervolting is indeed one of the last areas where manual tuning can still make a big-ish difference in increasing efficiency.
Yes and no.
The R7 2700x is an example of what you mention. Virtually nobody can get the 2700x to run all-core speeds higher than 4.35 GHz, and when they try, it becomes a monster of power consumption. Part of the problem is the 12nm process, but let's not get too far afield.
If you look at what a 2700x does with good cooling, it rarely boosts beyond 4.0-4.1GHz all-core turbo even with PBO2/XFR enabled. So shooting for 4.3 GHz all cores has a clear advantage if you are pegging all cores at 100% in something like a DC application and have the cooling and VRMs to handle the beastly power draw of hitting that clockspeed and staying there at full load, all day long. But for gaming, or something that may not use all cores all the time, a 2700x that is relying on boost (instead of a static overclock) can hit speeds of 4.3 GHz or higher on 1-2 cores, at least for a bit. Producing maybe higher performance in said game/application. So it's really down to what you are doing, even on a 2700x. There are situations where it is clearly better to do it the old-fashioned way and settle in on a static overclock. There are other situations where relying on boost is better.
I think with Matisse, we will see a return to static overclocks for maximum performance. AMD has advertised that PBO + XFR will get you maybe 200 MHz more than the max listed boost clock if you have very good cooling. So, for example, 4.8 GHz on at least a few cores from a 3800x. Can a 3800x go higher than that? Maybe. We don't know yet. But I think the potential is there to at least hit 4.8 GHz all cores static . . . assuming you can cool the thing while doing so. Boost won't hold a candle to that kind of performance in a wide selection of applications. It'll chew up a lot more power that way, though.