I have to get out and see a race, would love to shot some of these cars. I think they look good but I'm curious on why you stopped down the lens so far? Seems as though you could have choosen a better ISO level, mitigated the fencing, and blured the background better by using F8'ish.
I agree, I use as close to base iso as possible at all times and adjust aperture to achieve the required slow shutter speed for panning, only increasing iso when I cant get the combination of shutter speed and aperture I require. Try using F2 to eliminate the fencing . If possible move to a position where you can shoot over the fence, 1/160 of a second is generally as slow as I shoot to blur the wheels, although I am using much longer lenses. If you can, next time see if you can borrow or buy a longer lens.
This was taken at the official FIA test session in 2009 at Silverstone , the last chance I had to shoot F1 Cars.
"I say we take off and nuke the entire site from orbit, it's the only way to be sure."