I wouldn't worry (at all) about water ingress on the Tenere. Here in the Lakes we know a thing or two about rain (ask anyone who came to the Lakes in May), we do have (oficially) the wettest place in England just down the road.
I've had my bike upto the bottom of the fuel tank in water (not by choice but necessity), without any ingress. Also some of the worst rain you could imagine both here and in Scotland - You guessed it, not a drop!
Stop worrying and ride the bike
