Not sure if this is useful, but DPM stopped working for me once and was stuck
at 360MHz. I was doing some testing, Heaven had 60 fps originally and after I
run it again later, I only got 18 fps. Considering the lowest clocks I get with
DPM enabled are 300MHz, it wasn't completely underclocked.

It's definitely different from suspend to RAM, which pretty much disables DPM.
After suspend to RAM, I always get 280MHz or so. BTW, the same thing with
suspend to RAM also happens with Bonaire.