Well, you'll have your unrelated tasks (audio/physics etc) spun off, but you're still ultimately limited on how fast you can do the shadow volume extrusion calculations.
IIRC, the Quake IV multi-threaded patch just split-off the rendering thread.
Even so, you can still observe a huge disparity in performance depending on CPU:
Difference would be much lower at 720p because it's a higher load on the GPU then.