Well, too soft can be caused by several things... camera shake is one, auto-focus accuracy is another, heat haze/smog is yet another.
If you are using 150mm then cropping at somewhere like Silverstone where the cars are flippin miles away, its proabably the last two on my list.
150mm and 1/320th like those are should be perfectly possible to hand hold, but don't expect everyone to be sharp - I don't! Personally I pick my best shots for sharpness, not the subject - its nice to say you have a photo of the #1 man of the day, but if its soft for whatever reason, bin it is! My best shot might just turn out to be the bloke at the back of the grid...