@
Stankyjawnz: I can perhaps - can't promise it though - test that for you in the coming weeks, no idea when yet. However, not being able to achieve higher C-States, i.e. > C3, typically happens whenever you have high performance I/O chips in your system, be they for data storage or networking. You just introduce latency into your system, when the CPU continually enters deeper sleep states, which take more time to wake from again. The question is then how much additional energy saving you could in theory achieve by enabling deeper C states relative to problems introduced in your system. Most HBAs and networking equipment, such as SFP+ and SFP28 cards, where I know the problem from as well, are all built for enterprise environments, where latency problems are to be avoided. So you probably won't have any luck there trying to force something that is not designed for it.
However, as you can see from the datasheets - I haven't measured it yet - the newer 9400 and especially 9500 series HBAs use a lot less power than previous generations, so if you can upgrade your card, you'll perhaps save more power than what you would get from deeper C states - which you then also get if it is so designed.
9500-8i, 6W typical
9400-8i, 10W typical
9300-8i, 13W typical
Of course the max value is quite a bit higher. So newer chips are more efficient and I've stopped buying the older chipsets a few years ago.
EDIT:
The newer 9500 series chips, i.e. SAS3808 and SAS3816, are also found in, for example, Lenovo and Dell HBA cards.
I think it is the Dell HBA350i, Dell HBA355i, Lenovo 440-8i and Lenovo 440-16i. Those you might get a lot cheaper than the original Broadcom versions. Before buying read up on the SlimSAS low profile cables you'll need for the Dell versions.