Also check out this shader mod, makes me want to try playing minecraft again, but I rather not haha.
http://www.minecraftforum.net/topic/940974-125sonic-ethers-unbelievable-shaders-glsl-shaders-dynamic-shadows-more/
I really want to replace my GTX580 with one of these but I think I'll wait until 790 =p, and when I get a job again. Have to say though, this card looks amazing for those who don't want to use more than one pcie slot.
Also try streaming at a higher bitrate if you get a 2 pc set up working. I'm pretty sure that your hardware is having a hardtime rendering a higher quality stream while gaming at the same time.
Checking out the xsplit forums, the best way to have no performance loss while streaming is to buy a capture card and to have a decent laptop or another computer as the streaming computer so xsplit doesn't use up resources from the gaming computer.
I play the game on max settings (I think, playing the Korean version) with a GTX580. Before I edited the .ini it would run up to 80fps (hard cap) until I limited it to 60fps. I think it drops a bit sometimes but barely noticeable.