The overclocking capabilities
Essentially, if we're talking about the higher-end SKUs, there is basically none.
Based on my experience, the best case of scenario on 6C CCDs (3600, 3600X and 3900X) is around 4.25GHz, at relatively safe voltage levels.
In case of 3900X, given that you can cool the chip with two of those 6C CCDs. SKUs with 8C CCDs (3700X, 3800X and 3950X) the best case is around 4.15GHz. The 3950X is expected to be thermally limited, as a whole.
The biggest limit is the intensity (heat per area), secondly the voltage you can safely feed to the silicon. For example, the 9900K which has a reputation of being an inferno, has theoretical intensity of ~1.15W/mm² when operating at 5.0GHz (200W @ 174mm²).
Meanwhile Matisse can easily reach intensity of > 1.5W/mm² (120W+ @ 74mm²). The second issue is, that beyond ~3.8GHz the V/F curve becomes extremely steep. According to FIT, the safe voltage levels for the silicon are around 1.325V in high-current loads
and up to 1.47V in low-current loads (i.e ST), depending on the silicon characteristics. Because the stock boost operation is already limited by the silicon voltage reliability, the only way to eke out every last bit of all-core performance is using OC-Mode. Like on previous Ryzen generations, entering OC-Mode also means that you will loose the turbo boost (all cores operate at same frequency). On the higher-end SKUs, the single threaded performance penalty will be massive from doing so. For example on 3900X, you'd be trading additional ~100MHz all-core frequency to a loss of up to 450MHz in ST frequency by doing so. Personally, I advice against overclocking the higher-end SKUs at all, and instead increasing the power limits and trying your luck with the "Auto OC" feature (which most likely isn't beneficial).
The V/F testing was done using full resource utilization (FRU), meaning the stability was tested using 256-bit workloads.
Unlike Intel designs, Matisse does not feature an offset for 256-bit workloads. This means that to ensure the stability of the CPU cores in every scenario, they must be tested using this kind of a workload.
On Matisse, the delta in power consumption between the scalar and 256-bit vector instructions is massive, as expected (37%). That being said, there seems to be other design related factors limiting the maximum achievable frequency.
Despite significantly lower power consumption and therefore also lower temperatures, stability even in pure scalar workloads could not be achieved at much higher frequencies, compare to FRU scenario.
Performance per Watt
As expected, Matisse provides significantly higher performance per watt than its competition, thanks to its leading edge 7nm manufacturing process. Some of you might notice that Matisse's power efficiency seems to peak at 3.5GHz, despite the fact that semiconductors do not behave like that. The reason behind this was revealed by Vmin testing, which clearly illustrated that Matisse lacks fused V/F (voltage-frequency) curve below 3.4GHz. This means that below 3.4GHz frequencies the voltage is always at the same level, it is at 3.4GHz. The stock (fused) V/F curve appears to be extremely well optimized as well, leaving only the temperature factor on the table.