I tried this out last night, and loved it. My in game FPS almost doubled, and my stream (once I worked the kinks out) looked a lot smoother.
Here's what I found:
-Using their version of Screen Region for TF2 caused a massive stream FPS drop. I'm not sure why, but my stream hovered around 15fps instead of the 30 I had it set to. using their program capture bumped the stream FPS to 30, and did not affect my ingame FPS at all. I'm not sure why OBS couldn't handle my screen region.
-If you're layering something on top of your game footage, such as a webcam, a smaller screen region, an image, etc, know that the program is currently backwards. The "top" layer is actually the bottom. If you have a webcam, and you select "Move to top" it will actually put it BEHIND your game footage. This actually had me thinking that my webcame wasn't working.
-The stream bitrate doesn't seem to be variable. In Xsplit, you could set it high on the off chance that your stream would spike to that rate (however unlikely). OBS will try to use that rate regardless of whether it needs to or not. I usually had my rate set to 3500 in Xsplit, but that was causing some video issues for people who had connections or computers that couldn't keep up. A rate of 2000-2500 is plenty for a 720p stream.
Hopefully this will help a few people who are running into the same issues that I had.