Back to the hammer spring...
I have done extensive work inside my Super Redhawk. It's got the same action as the GP. Despite removing as much friction as possible, when I ran the heaviest Wolff reduced power hammer spring the gun would not make it though a full cylinder. I was running 45 Colt primed with Remington, Winchester, and CCI. None of them made six consecutive single action shots. Double action was understandably worse. Sometimes a second hit would light the round. When that failed I could drop them in my Redhawk or Vaquero and light 'em off on the first hit.
When you take that thing to the range be sure to include all four hammer springs and your tools. You're going to need them.