Yes, being under a covered stand does make a difference to the amount of light getting into your camera depending on how deep you sit within the stand so you may find yourself having to either change your aperture or ISO setting or use an ND filter. However, I don't think that difference puts you at a distinct or insurmountable disadvantage as variable weather conditions will probably force you to change your settings whether you're under a covered stand or not.
I was at Silverstone myself last year in the International Pits Straight Stand on most of Friday, the Becketts Stand on Saturday and in the open at Stowe on Sunday. As you know, there was a huge variation in weather conditions ranging of light to very heavy rain to overcast to bright and sunny with patchy clouds so I was changing the aperture frequently - the shutter speeds varied just to work out how slow I could shoot to give a perception of speed as I'm still a relative beginner with motorsports photography.
Have a look at my exploits from Silverstone last year:http://www.flickr.com/photos/59694573@N ... 509616924/