400# could largely be taken up by the supercharger/intercooler/water tank setup they are going to run. It's probably going to be a water/air intercooler with heat exchangers in the intake manifold, so the IM will probably be revised (heavier) also. And then there's the added weight of a DCT compared to a manual.... They end up at 4,200# because the Mustang already starts out as a porker.