Doh, The track pictured is for the state race. clearly not a s-shaped track. Why would you want the weight in the middle ?
The flat is so long that the rear weighted cars momentum falls off towards the end of the track. The more center-weighted cars are beaten to the bottom of the hill, but end up passing the rear weighted cars before the finish line.
And after sneaking a peak at the underside of the 1st place winners in a couple of classes, I saw that they were indeed more center-weighted than rear weighted.