it's not very hard to make something use all processing power
LOL even with gpu it's very simple
it all depends on your update speed. (untrotled always sucks all power)
but feel free to differ. doing opengl would be interresting for me as well.
I only used it once.
Take for example old games. (like C&C) those didn't have gpu as well. (just like the sdl without opengl case) that game had a decent framerate i would say. So it all depends on your skills)
if you can always later add gpu specific drawing fucntions i think.
I think it's best to use a library as sdl as that one has a simple interface to all outside devices (timer/sound/keyboard/mouse/joystick/video)
optimizing can be done in a later stadium.
But I'm only giving some advice based on experience. So do what you want with it.