It did have a purpose other than pure styling. Extra space was needed for the new sealed central air system, turbocharger and electrical cabinet to all fit behind the cab. This was gained by raising the height of the locomotive above the height of the cab.
Whether or not a 30 has dynamics, it has that roofline. The only way I know to tell them apart is that a non-dynamic GP30 doesn't have a fan in the fairing area. However, you can't see this from ground level...
