I had that very same combo at that point in time. The Sigma was / is a good lens for the price. I still have mine, but my wife has the camera (although she uses it very rarely). I'm seriously thinking about (at some point) replacing it with a used Canon 24-70 f/4L IS. For landscape work I almost never get below (above?) f/5.6 and L glass / coatings will be better than the Sigma (plus the IS will be nice for hand-held shots). But the Sigma honestly still isn't a bad lens as long as you don't need light-speed focus, it just doesn't have a lot of snob value.

About my only legit complaint was that for manual focus it would benefit from a little less sensitivity (i.e., the ring should turn more from near to infinity).
I like the shot and it looks like a very interesting location. As to the pano artifacts, I've seen quite a few in Google street view imagery. Granted, that's hardly artistic photography, but it's funny to occasionally come across a vehicle with doubled front (or rear) wheels, for example.