I've ridden my cyclocross bike with mountain bikers before on fire trails. And even with 32mm dirt tires, my bike was much faster: and by this I mean that downhills, when everyone was just coasting and no one used their brakes at all, I'd pass many mountain bikers. And this is with me on the hoods - if I was to get into the drops, I'd have been even faster. In other words, the aerodynamic advantage makes a huge difference.