The VS stands for valvetrain safe. Many people find that the springs break when they put this cam in w/o changing them. I highly recommend changing the springs and retainers, and it's easy when you have the motor out. As long as you do that, XP/S1X are both easily doable. Both would give you a little more than the VS on the exact same mods.

The front exhaust manifold is holding you back. It necks up pretty nasty. That can be fixed with a power log or other free'd up front manifold.