If you are going to be pouring them yourself, I HIGHLY suggest looking up White Label Lubes and giving some of their BAC or Carnuba Red lube a hard look.
There are others out there, but for the price and performance these two are hard to beat for what your looking to accomplish.
If you are buying commercial bullets, then you should at least slug your bore to determine the best size to purchase. Some commercial casters will do different sizes and some won't, and in the cast game fit is King and lube is Queen. Even with the GC designs, you can and will experience leading to some degree without the proper fit and lube for the job. Slugging can easily be done with a pure lead sinker that is a bit over your bore diameter. Just be sure to use some oil and a substantial rod to drive it through with, and not a cheezy wooden dowel.
Don't let this distract you from your project, just do a bit of homework and you will be fine. There are folks pushing cast 2K+ FPS all the time, but they have things dialed in to fit and lube their barrels. Quite a few of them are also running lower BHN alloys as well . Like I said, you simply need to assure your fit is good first and foremost, and then the lube, and the rest will take care of it's self.