I have had a fairly good read through this thread although not totally comprehensive.
Just a few observation that might have slipped past the radar.
When buying lens, anything that has f4.5-5.6 is going to be slow at focus. I have a 80-400 f5.6 Nikon and it can be a dog when I want fast focus. That is even when using it with my D3.
For future use you should look at lenses that are f2.8 if you want fast focus and response. These lenses of course cost a lot of money. The f5.6 lens cost a lot less. A slow focusing lenses is not crap, it is just more budget related.
Sigma like Nikon, Canon and Tameron make fast and slow lenses. If you buy a zoom you are assured that they are not a master of any focal length and are in fact a compromise. There are fast zooms but again they cost a lot of money. To be honest only spending a K or so is never going to assure you of getting a fast lens. Your only mistake was to buy without seeing how fast the lens was. Many photographic retailers will let you mount the lens on your camera and try it before buying.
Added to this is that you have a prosumer body. Pro bodies make slower lenses faster in response, but virtue of the features the pro body has. They don't make them into a silk purse but they seem better than owning a sows ear. Prosumer bodies are like slow f ratio lenses. They are budget related to allow the average punter the ability to enter the market.
Sigma is not at fault here. You made the mistake without finding out first. I am sure the lens is fine, you might have to assess what you are going to use it for now. Good work can be done with slower lens, just try to find its strengths.
This has always worked for me. Take a look at my Earth section in website in my sig. Many of the shots are taken with slow lenses.