I have the LS Cloud and it does work noticeably better than standard tupperware. I use it on a 580 EXII and have never had a problem with power. Also, the weight has never been an issue for me. ... But, it is BIG. It may not look big, but trust me, it is BIG.
Remember, your 580 EXII is already BIG (substantially bigger than a 430). Add a LS to the head, and it gets even bigger. It draws a lot of attention, and people tend to stare at it. I quit counting how many time people have asked "what is that thing?"
I still use it, because it really does work well. ... But since I'm not an "event photographer", I get annoyed by all the attention.
For point of comparison, I get more stares and comments when I'm shooting with the LS than I do when I'm shooting with a big, white 70-200 L2.8IS