I used to shoot Gold Tip and had problems with the arrow doing funny things on impact with elk. Quartering away shot behind the front leg came out in front of the hind quarter on the other side, kinda did a uturn inside the elk. Had a similar thing happen on a deer in the late season. I switched to FMJ and haven't had any issues. Maybe it was just a quirk twice but like I say no issues since switching to FMJ.
As for going to a lighter broadhead to gain speed I would be concerned with getting the arrow too light on the nose. I personally think my arrows fly better with more weight in front of the center of my arrow. If you are going to change your setup you may have to tweak a few other components as well to get your arrows to fly right, arrow spine, broadhead type, weight, brand and fletching type and length.
Talk to your pro shop they will have a ton of information on what they have had success with.