Could be FS, I guess, depends what level performance pack n such.
If you're new to autocross, new to the forum or need a hand with what class you should be in, ask for help here.
Maybe I missed it but I don't see an Ecoboost Performance Package Mustang listed. The base car is DS, but in Mustang and Camaro lines, the "track" option cars are typically classed higher than base model. An exception is the 1LE Turbo4 Camaro, but that exception is specifically listed in the rules. Not seeing the same exception for a Mustang Ecoboost Performance Package called out. So I don't know. But it might be interesting. I'm told the Ecoboost PP is a 4.5second 0 to 60 car with 9.5 wheels. And autocross was one of the design priorities.