What I did was enable "ForceLowPowerMode", which is supposed to clock down to 1/2 (300/200MHz) core and memory. I edited radeon_pm.c so "ForceLowPowerMode" would downclock to 1/6 (100MHz) of the original frequency (600MHz), and that saved me like 4 extra Watts on my mobility HD3650.
I also enabled "ClockGating", in case that saves a little extra power.
You seem to not be reading along, as what you suggest is only a quarter of what I've already done.
DPMS mode minimizes the various clock speeds (core and memory) and reduces the card to using a single PCIe lane.
Even still, the card is running and powered on, and in my case as well as many other's, that is not necessary.
I was reading along. The bios also has settings that let you edit the power saving modes though most cards don't provide enough voltage settings to do much good. Many chips will run fine on much less than they are set to as they are set to run for worst case scenarios. Computer manufacturers want to be able to turn off discrete gpu's when they are not being used you are just trying to get there before the cake is baked.
It's going to take a while to do it. If it could be done it would be a feature in catalyst and nvidia drivers right now. Get a 4770 or 4670 and sell the 4870X2 or wait and wait and wait till shutting off video cards or even one of those cores on that X2 becomes possible.
When I was in college I studied psychology and mechanical engineering. I used to tell my psychology friends we are all engineers some of us just don't know it yet and I used to tell my engineering friends we are all psychologists some of us just don't know it yet. When solving problems in the physical world sometimes you have to let the physical world engineer you. There are always limitations and always competing desires and you have to make choices as to what is more important. Massive gpu horsepower doesn't have an off switch yet.