I wouldn't treat this as a guide, as I don't fully understand myself, but this is what I did in order to increase the limit to 25W on my 330S
1) Install Clover bootloader to an empty USB key (https://sourceforge.net/projects/cloverefiboot/)
2) Install AMD uProf to test the outcome (https://developer.amd.com/amd-uprof/)
3) Enter the bios on your machine and move USB FDD to the top in the Boot Priority order section and disable Secure Boot from the security tab
4) Boot into Clover from the USB drive
5) Once in the clover menu, press F4 to export your machine's current settings to the USB drive (this takes a few moments).
6) There should now be a file on the USB drive /EFI/CLOVER/ACPI/origin/DSDT.aml This will need to be decompiled and edited (I used MaciASL)
7) This step is where it varies between machines. I found that searching for the _PSR method and adding the code described here worked for me. https://www.reddit.com/r/Amd/comments/a2vs55/i_successfully_disabled_stapm_and_increased_the/eb7xngs/
The value 0x55F0 = 22W. I changed it to 0x61A8 which is 25W.
8) Recompile and save the DSDT.aml file under /EFI/CLOVER/ACPI/WINDOWS/DSDT.aml
8a) Delete the origin folder (Not sure if this is strictly necessary?)
9) Boot from the USB and at the clover boot menu press enter to start windows. If all is well, Windows should start.
10) Launch AMD uProf and select "See what's guzzling power in your system". Select the controllers tab and tick the 4 options under Controllers Counter Selection. You may also want to tick the 2 options under Power. Click start profile. You should see the STAPM graph and critically the Socket0 STAPM limit should now read 25W instead of 15W.
11) For further verification you can run Cinebench CPU test a few times and observe the throttling to 15W behaviour is now gone.
There does already seem to be some STAPM code in the 330S DSDT, with a big section of if / elses that set various limits. As far as I can work out, this section is what determines that dynamically adjusting STAPM values when running on battery and for me changing any of these values had no effect when running plugged in, only when powered by battery.
So it seems that the 330S is a little more complicated than other laptops, but the above steps worked for me on mains power, and I left the settings the same for battery as battery life is already very short on the 330S without increasing the power consumption.
Thanks to the guys over on Reddit and this forum for their hard work on discovering all of this. Such a simple fix turns this laptop into one that performs surprisingly well.