I know this topic belongs in the 2015+ section! but the threads there haven't concentrated on this point! and I think it's relevant to this group. Car and Driver is saying that the Trinity 5.8 we've grown to love will live on, at least initially, in the S550 platform.
2015 Ford Mustang Leaked! 360º View and Full Details – News – Car and Driver
2015 Ford Mustang Leaked! 360º View and Full Details – News – Car and Driver