Is the Grand Prix GTP supercharged? I assumed you were talking about an s/c pulley, unless you were talking about underdrive pulleys. If it's an s/c pulley, then generally a smaller pulley will spin faster and yield more boost, but I don't know what specific size you would need for an increase of X amount of boost. If it's underdrive pulleys, than those are usually application-specific, so he would need to get a set that's made for his engine.