It's been posted elsewhere that the bare Pi uses about 330 ma when idling and with nothing else connected other than HDMI (which draws no current, so all further examples include HDMI). With a keyboard, mouse, and network connected the current draw of the Pi is about 385 ma, and with keyboard, mouse, and network connected and the ARM CPU running at about 99% of load, the Pi draws about 450 ~ 550 ma. That leaves up to 250 ma for various levels of GPU load and/or additional minimal-load USB devices before hitting the 700 ma maximum that the on-board components and printed-circuit board traces can carry.
The BRCM2835 SoC used in the Pi (same as the Roku 2 models) automagically performs GPU power management dynamically based on usage and, although it can be overridden in the /boot/config.txt file, it then runs full-bore no matter how much, or little, computation is being done on the GPU.
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!