Dragging up an old thread sorry, but I think this will benefit newcomers with what I have learnt from experience and advice.
I dont agree that 300mm is not enough for Silverstone, it has been pretty successful for me, sure a tc will help but ultimately its depends on what kind of shot you want...a close up of the drivers eyes as they turn into Stowe isn't going to happen, but if you are realistic you can get pretty good shots as long as you choose your location well.
I've posted links to large images on a few so you can actually see what is possible - i'm using a 40D for these so only 10mpx so most bodies now should capture some extra detail compared to these as well - all from spectator areas-never been media. all with sigma 100-300f4 non dg (No stabiliser) a £400 to 500 lens and ONE with a 1.4x sigma non dg converter (£60-70).
total gear value of about £600 max secondhand, so this is what you could do without spending £1000s on more reach.
40D @ 300mm - no TC @ Luffield complex grandstand - so behind a raft of spectator terracing as well
British GP 2009 by
Fireproof_Art, on Flickr
40D @ 300mm + 1.4x converter @ at exit of brooklands pressed up against the fence

(the full size image is 3 times bigger than this and quality is great)
8 by
Fireproof_Art, on Flickr
40D @ 300mm no tc from inside of stowe - vale. behind fence again but not always accessible for big events.
Brundle WEC 2012 by
Fireproof_Art, on Flickr
40D @300mm no TC from the loop (between village and wellington straight) shooting through vacant photographer hole - again not always accessible.
BritCar 24hrs Silverstone #88 by
Fireproof_Art, on Flickr
40D @ 300mm (NO TC!) from club gold grandstand - granted this is a pretty large crop as I had whole car in frame, only a 10 megapixel sensor though
Sebastian Vettle by
Fireproof_Art, on Flickr
And another one from Club - 40D 300mm no tc - i'm in the middle of the grandstand as well.
Jaime Alguersuari... 2011 Silverstone by
Fireproof_Art, on Flickr
Hopefully this will give beginners a better idea of what is possible and a better insight into whether they need to have gear envy or not. I'm not saying my photos are artistically/compositionally wonderful - this is how close you can get for under £700
