How much weight gain from adding a Dana and roll cage.

Can you define "much more"? One of the car mags did an article years ago and yes, the Dana and 12 bolt needed less torque than the 9". But, if it was a substantial amount, you can bet the guys running the fast heads up stuff would be running Danas and 12 bolts and the aftermarket support for them would be there too.
Seems like with all the fab 9 producers and part availability that might be why people choose the 9 over dana