Maybe, I suppose the Focus is a bid stiffer. The Rebel and the 15-85 is not very heavy, but a bit front-heavy.
But sometimes it is not the stiffness, but some play between gorilla and structure, so the focus wouldn't help either. And then the Focus is also bigger.
I think I'll keep the SLR Zoom and buy a real tripod sooner or later. I did buy the Ballhead X which has an Arca-Swiss quick release so that I can mix and match with future tripods. I would always recommend the X over the Ballhead they offer in a bundle with the SLR Zoom for that reason.