I haven't used the sigma lens so I can't comment on it but I shoot with the 70-200 F4 L and i'm really happy with it, when I have the money it will be upgraded to the 2.8 not because i'm unhappy with it but for the indoor work I do the 2.8 would be a big help.
The cars going faster at Mallala can work to your advantage as well. It means you can shoot with a slightly higher shutter speed and still get a nice background blur. A lot of my drift photos are shot at 125/160. When I do shoot lower then I go a lot lower like 30/60th but I throw a lot of those photos away.
Photoshop is good but if the focus is a bit soft to start with then Photoshop can't fix that.
You might want to look into Adobe Lightroom. All of my drift photos are done in Lightroom not Photoshop.
Hope this helps