Running stacks wont reduce egt's. Big Rigs run stacks because the exhaust has no where else it can be routed.
Fuel causes heat. The more fuel the higher the heat. That's why programmers and tuners raise egt's. Higher line pressure and more fuel. Stock turbos are one of the biggest issues for heat. If you want to reduce egt's there are several things you can do. Bigger intercooler, bigger intake elbow, bigger manifold(s) bigger turbo water/meth injection. If your an unfortunate one that has a corked up truck do to emmisions equipment you can delete those(egr, dpf cat). Then you will have warranty issues, though.