I currently have a tamron 70-300 f4-5.6 and a sigma 150-500 f5-6.3 im defiantly getting a 70d in a couple of months I will just stick with these for now then
With a shot like this with very little light from external sources like a following car or floodlights even a 2.8 lens and massive ISO won't really help you.
In my limited experience and looking at other togs work you need to have factors on your side. Even at floodlit races like f1 at Singapore they struggle with slower glass, you probably need to adjust your expectations for what you can achieve with these kind of lighting conditions
In your example image, I don't see where a faster lens or better ISO capability will help much. The image looks fine. If you gathered more light the bright areas would just blow out.my pics are ok for the lenses and 450d I have at the moment but they cant really handle it I just want to achieve good quality images