Experts? You mean McExperts?
I doubt their statement is correct. The Space Shuttle burns it's main engines while it flies to orbit. Your shuttle will not. That means that if you want to prevent the whole stack to tumble, you'll need to deflect the engines to produce thrust that points right through the center of gravity. Having a 15 ton payload + weight of space craft on the side is a bad idea when the space craft does not burn it's engines... the center of gravity will thus be far off the center of stack... because of this, to fly upwards, the stack will have to be angled a few degrees relative to the airspeed. That will cause considerable profile drag and turbulence!
If you put the shuttle on top, you'll be able to fly right into the windspeed, minimizing drag. The problem then becomes instability... wings will start acting like canards. Even a small deflection that cannot get corrected by elevons will topple the space craft over.