If you change the calipers and rotors to multi-pot/larger units you'll also need to either disable or modify the ABS system and add an appropriate master cylinder. Don't forget the proportioning valve either.

Generally the VX has plenty of brakes. I've rallied it pretty hard on both asphalt, gravel and dirt surfaces and there's no issue with stoppage.

So if you are having problems stopping, you have a problem. Correct it before you become more familiar with certain physical laws.