There's basically three separate problems here, none of which is getting solved in the way you want.
System hibernation to HDD is cheap and easy to implement, but it's a massive security mess (you're reading unencrypted code and data from an insecure medium), relies on free HDD space, and does nothing that having quicksave/quickresume as part of TRCs doesn't.
System hibernation to onboard flash is more secure, quicker, but nowhere near overall speed benefit of using the same flash as a cache, like the OG Xbox used its HDD. Also it's expensive on the scale of around a billion dollars plus.
System sleep is cheap, mostly secure, probably not too hard to implement the software side of, but involves leaving the RAM and associated hardware turned on while shutting off the CPU and GPU. However, if you're pushing the heat and power envelope to begin with, leaving major components turned on indefinitely isn't a great idea.