+1 for this idea, and maybe even a system to make the size variable in the skin file.
And in real life they have huge banners on the windscreen AND helmet. The only racing with a high angle going upwards would be mountain climbing, and I'm pretty sure the GTR's won't be used for those tracks...
Ohh, another reason why a bigger banner would be nice is because of the sun. I can't imagine that the people saying -1 would like to drive 250 kmph and have a bright sun in their face.