You do realize PBO is an optional feature right? And it can actually drop your single-core clocks/performance if pushed too hard due to thermal reasons, and ESPECIALLY by setting everything to basically infinite. (It can't stay at max single-core boost as long due to the increased voltages). The secret to PBO is manual tuning of the parameters to maximize all-core boost w/o negatively impacting single-core. Definitely takes some work & time to find that sweet spot though. And many single-thread processes have super short spikes in load, so a short time at max boost IS definitely still useful vs not having that boost at all. Also, if you're running the latest AGESA version (1.0.0.5) it'll hit the max boost on a single core pretty regularly, especially with a good undervolt.
And the stock all-core boost for the R9 3900XT will be around 4.3 - 4.4GHz (up from 4.0 - 4.1 GHz), meaning a 4.5GHz overclock is totally within reason (not that I would do it though, ≈4.35GHz all-core + 4.8GHz max single-core boost > 4.5GHz all-core with no boost). That's not where the biggest performance gains from the refresh are going to come though. It's the +200MHz to the maximum Infinity Fabric clock that's the total game changer. Overclocking a Ryzen 3000 chip to like 5GHz on LN2 barely improves gaming performance after like 4.4 - 4.6GHz or so, due to major core-to-core (specifically CCX to CCX) communication bottlenecks. Running 4000MHz RAM with a 2000MHz F-Clock will essentially unlock a new gaming performance tier Ryzen's never been able to hit before.