I was asked to keep the panniers on as the shoot had to demonstrate a Sportsbike, Commuter and a Tourer. I took the corners VERY carefully, but was OK opening up on the straights.
What I should have done was remove the luggage and do a lap to see how much difference the air-braking effect was making, but it was so wet, I just couldn't be arsed.
I can report however, that with full luggage on, keeping the revs mainly above 5000 rpm and red lining on the straights, I still managed 49mpg!!