That's odd though, from my testing using OBS own capture gave me better performace.
180 fps on Zeno Clash with OBS capture and around 100 with OBS+Dxtory.
Maybe OBS just has a particular issue with whatever game you were trying. Some don't even work at all.
It very well could have been the game, but Far Cry 3 is the only one I've tried with OBS so far. I can tell you that I didn't have much success with PCSX2 last night (using dxtory) and OBS. I had audio sync issues there while playing the game. Changed over to ffsplit with dxtory and the audio was perfect. So, it seems like I'll be switching between these two until one of them is far enough in development to be able to avoid these issues.
OBS had this deal where it would disconnect and reconnect to twitch every few seconds. Don't know if that was a setting I unknowingly messed up, but it caused me to switch to FFsplit.
FFSplit works fantastically. The quality isn't quite where I want it, and I have to play games in borderless windowed mode to get it to work, but otherwise it's pretty good.
I also like ffsplit, but I think its interface could use a bit more polish as it still feels very "thrown together" by its appearance. As was recommended me to me, give dxtory a whirl with it as the quality will be a bit lower if you just use FFsource or the FFsplit overlay.