While this is for the 12900 they do have the same cores...
Not quite. Raptor Lake enlarged larger L2 caches, among other things.
If you want to know how these CPUs perform on a restricted power budget, the best way is to
actually measure it. ComputerBase did this with the i9-13900K and found its mulithreaded performance (on their multi-application suite) to be exactly
50% as much, when limited to
45 W, than the default PL2 of 253 W. Using a PL2 of 125 W, as the reference point, 45 W performs 60% as well.
Intel Raptor Lake im Test: Leistungsaufnahme und Effizienz / Leistungsaufnahme in Anwendungen ab Werk / Der Verbrauch steigt
www.computerbase.de
It's a shame they didn't use the 35 W threshold of the T-series, or repeat the testing with the i9-14900K, but we know the i9-13900 and i9-14900 dies are the same stepping, and at least the 45 W figure gives us some real world data to consider.
tau only works if it is enabled,
The only chance you have to disable it is on unlocked, K-series CPUs. On these T-series, the most you can do is to reduce it from the default of 28 seconds (which is counter-productive, from a performance perspective).
For those who don't know, Tau is the maximum amount of time the CPU will boost at PL2, before dropping down to base power (PL1 / TDP). Manufacturers can restrict it beyond the CPU's defaults. On the Dell compact desktop I use at work, they cut it down to just 12 seconds. You can tell it's Tau-limited and not thermally-limited, because thermal-throttling is generally more gradual than Tau-throttling. Also, the CPU temperatures drop under Tau-throttling and yet it still doesn't let the CPU run any faster.