Thank you! Pump is ACDelco.There are MANY opinions, rituals, and practices when it comes to anything cars. Cooling is no exception. In my experience, a clean and well designed system always works well. And in my experience, when that isn't true, then something else is wrong.
I've heard that water pump impellers make a big difference with Pontiac. Are you using a good, name brand pump, or a parts chain rebuild?
As for the thermostat, bypass holes shouldnt make or break your system... However, as I mentioned, when all else is right, they can add to efficient operation. It's not likely that they will cure your issue.