It's very likely that, yes, PBO just has no headroom. On a single core we're already seeing 1.5v+ (my 3600X hits 1.469-1.475 to reach its 4.4GHz) to hit boost clocks (have seen a few screenshots exceeding 1.5v also), it's likely there just isn't a satisfactory trade off to get any extra frequency, and the safe voltages for all core loads are touted to be around 1.37v max.
Temps seem to be pretty limiting this gen as well; my 3600X during gaming sees 84-85C and my setup goes as such: NZXT H700i w/ EK 3.2 DDC elite pump with 140ml reservoir -> out to Radeon VII -> out to XSPC EX360 radiator with 3x Noctua NF-A12x25PWM -> out to EK supremacy block on the 3600X -> out to XSPC TX360 radiator also with the NF-A12x25's -> back to pump. My fan and pump PWM profiles are pretty aggressive as well, fans are at 2000rpm with CPU temp above 65c and pump is at max rpm (4400-4500) above 70c.
Regardless, AMD has done really well with IPC for this gen. Seeing some reviews with clocks matched, Intel can't keep up, that overclocking headroom is the only thing they have to save them, and that only really works in certain scenarios. It's a good time for CPU innovation and evolving; now we just need that to come back to the GPU front.