I've had a cat convertor go bad on me before (not on my ecoboost) and it would rattle. The cat's internal honeycomb structure melted a bit and a chunk of it broke off. When you hit the gas hard the chunk of loose honeycomb material in the cat would roll to the back end of the cat due to the force of acceleration. Once it rolled back it blocked the exhaust from leaving the aft portion of the cat causing immense back pressure. I can only imagine the amount of back pressure there would be if this happened on a turbo equipped car! Could that immense back pressure cause you to blow a head gasket? Maybe... If I were you I'd remove the down pipe and shake it... do you hear something loose and moving around inside?
2016 Ecoboost Mustang
2009 C6ZO6 - Katech Stage 2 #115
2005 Mustang V6 (Sold)
2003 Terminator (Sold)
2002 Mustang GT (Sold)