Who is saying that 5800U behaves differently? And 45W is obviously the turbo limit for 35W Cezanne chips, this should be obvious from the graphs that have been linked. I suppose that's what you have?
Anyway, since you own the hardware, could you do us a favor and run a couple of experiments? If you limit the maximal power consumptions (not sure how to do it with AMD CPUs, there must be some sort of utility) to 15W and run some benchmarks on your 5900hx, what kind of results do you get?
I have a 5800-H ( 8 cores - 16 threads)
just done some CB23 runs
few caveats, I currently have 322 processes running including ~ 4 web server frame works, ~ 16 connections open to mysql , teams , webex, etc, so my idle is hardly idle ( my idle is about 6-8% CPU utilisation @ 3.5ghz)
also remember lots of these laptops have junk memory , mine is 16GB 3200 22:22:22:52 , for example my cheap desktop memory is 3600 16:19:19:39
13watt CPU
multi core clock: 2.1ghz
per core wattage: 1.7
multi score: 6418
pinned to a single core
single core clock:4.1ghz
single core wattage: 7 watt
single core score: 1202
15watt CPU
multi core clock: 2.27ghz
per core wattage: 1.9
multi score: 6856
multi core
NO SMT* clock: 2.6ghz
per core wattage: 1.9
multi score: 6573
pinned to a single core
single core clock:4.15ghz
single core wattage: 8 watt
single core score: 1253
25watt CPU
multi core clock: 2.7ghz
per core wattage: 3
multi score: 7494
multi core
NO SMT* clock: 3.05ghz
per core wattage: 3
multi score: 7083
pinned to a single core
single core clock:4.15ghz
single core wattage: 8 watt
single core score: 1253 ( temp limited)
unpinned single core
single core clock:4.15ghz
single core wattage: 8 watt
single core score: 1283
35watt CPU
multi core clock: 3.1ghz
per core wattage: 4.3
multi score: 9225
will run higher wattage tests and update
* via affinity so still 16 processes running
ZOMG amd SMT improves PPW ..... intel P cores in shambles