The latest build of VCM Suite 3.0 has a RPM Limit Patch for the 2011-2014 Mustang GTs. This patch will raise up the hard limiter to around 8200 RPM or so.
This option is available in the OS tab of VCM Editor.
Once it is applied, you will need to set the rev limiter to where you want it.
Typically these cars run into a hard limiter around 7700 RPM. With the patch applied we have tested multiple Mustangs and have gotten at least 8150 without issue (both auto and manuals).