70-200 2.8 is a staple lens and very versatile not just for motorsport. The Sigma lenses are very good. Digital Photographer recently tested the Canon, Nikon and Sigma 70-200 2.8. Now the Sigma did come third but it was quite close to the Canon They made the point that it was excellent value for money. You will lose the IS of the Canon but at least it wont be white.
I am surprised it doesn't come with Sigmas own OS system it may do in the future but they seem to add it to odd lenses.
It also largely depends if 200 is enough reach for the motorsport you shoot. I would still recommend getting a 70- 200 first and then think about replacing the 75-300 with a faster 300 F4 or F2.8 depending on your budget at the time.