Looking forward to this feature once it comes out. My problem is purely bandwidth-related; simply put, I can't output anything greater than 360p@30fps, even though my computer can very capably output 1080p video.
In the interim, I've set up something rather clever if I say so myself using multiple OBS instances. First, I have one instance set to record at the resolution I want (say, 1080p), and set the preview screen to 1:1 mode. I then open up a second instance that records the first one, specifically through Window Capture mode and selecting precisely the Sub-region which is the preview.
I actually just thought it up a few minutes ago and tested the setup now. The results are seamless - Twitch is outputting exactly what I want it to stream (360p/30fps), whereas the recording was saved at full HD settings (1080p/30fps). There is no perceptible delay either aside from the one introduced by Twitch.
The only downside to this setup is that your system resources will be eaten up by the cost of two separate OBS instances running at once. If you're planning on playing a 1440p game and running 2 video streams at 1440p and 1080p, that's going to gobble up your resources like nobody's business. Until we can get a much more efficient and built-in option from the devs, though, and for as long as your hardware can handle the load, the 2-instance workaround will be a suitable solution.