I redid my 11GT. Here is how: Disassembling a 2011 Mustang GT for a stereo install
I changed the fronts and rears, added a Stealthbox to the trunk, and added amplification for the whole works.
Stock head unit, you cannot tell anything was changed (until you turn on the stereo, then it's pretty obvious
You can see the final results of the remaining trunk space here: 2011 Mustang GT
Don't lose much mounting the amps like this.