With chambered pipes the overlap will pull heated exhaust instead of cold air, it's going to scavage unless you run a no overlap cam, and I think the only one I have found is made by Federal Mougal for Ford small blocks.
Without backpressure you change the balance of flow thru the engine - i.e. over porting on the exhaust side will kill flow into your heads, same same with open headers except when at WOT.