Well the berzerkers come with running legs so they are more dynamic as the normal
csm.

And you can do a lot with changing the arm position and alignment of the torso. Especially the arms are in need of repositioning as they sit a little bit to close to the torso. Just pin them to the torso on in a different angle and fill the gap with
GS.
Those two are converted but you could archive similar poses only with slight reposing of the bits from the box.
And if also seen nice kharn conversions based on this model. It's an older metal one but still workeable:
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat440108a&prodId=prod1050011 And if you just want to make them look different there are a ton o mean looking chaos bitz(weapons, shoulders and heads) from
puppetswar in their mayhem warriors category.