Maybe technique rather than tools is the answer there sir

A 100mph car isn't that fast if it's 200/300 yards up the road, it's really fast when it's only a few meters from you. Think about it, a plane flying at 500mph doesn't appear that fast in the sky because it's far far far away, however, a 40mph car a few feet from you is actually pretty fast

Unless you are having privileged track access, I would be surprised if you were that close to the car in question, particularly since you are after something around 300mm which is 450mm in real terms, and therefore quite a long lens.

So, technique, what most photographers do in this instance, is manually focus on an area where the car will be, kerb, tarmac, barrier, etc... and shoot when the car hits the "in focus" spot, rather than try and track it with auto-focus. This also enables you to be more creative with your framing, rather than have the car slap bang in the middle of the frame.

Lastly, autofocus in most cameras uses image contrast to focus, so if you are trying to focus on a plain colour panel, no amount of fast focussing motor will help. A lens with a max aperture of F5.6 may also not provide enough light for the autofocus to work fast enough (identifying contrast) in overcast conditions, again, nothing a HSM/SSM/USM motor can fix.

Hope this helps, maybe saving you a few £££s in the process