disclaimer: I'm not a seasoned wirecast user so all could be up to my limited understanding of the software and streaming but...
Skype gives me best picture but... It crashes regularly... Another thing is that when canvas is 1080p you don't get the full image in skype(sides are cut off) --> picture below is streamed at 720p
Google+ simply doesn't crash... but it has less image quality... Picture below is streamed at 1080p
When it comes to streaming 1080p with 3 HD camera's via 3 ultrastudio mini recorders using thunderbolt and a new mac pro. The pro handles it very easily. CPU load is about 30%. I had photoshop open as well. RAM was no issue at all.