Actually simpler than all that above. Change line 145 of SFEMP3ShieldConfig.h from

#define MP3_DREQINT          0 //Corresponding INTx for DREQ pin

#define MP3_DREQINT          1 //Corresponding INTx for DREQ pin

This will swap the interrupts and should match the Leonardo.

If that works I can put in a Flag to auto select the correct value based on board type.