In all fairness, the Guide Gun was not made as a 200 yard gun. Sure, its capable, if you spend many many hours at the range getting to know ballistics for a particular load, and you have the marksmanship skills to do so with open sights. Most hunters simply do not have what is required to cleanly take game at 100 yards with open sights let alone 200.
Even if you mount an optic, the Guide Gun is still designed to work primarly as a deep cover gun. Im not saying it wont perfom at londer distances, but in the majority of circumstances, it is the shooter that is not prepared.
So, to answer your question, the .45-70 is VERY capable to cleanly take game at 200 yards regardless of what model you choose, providing you put in the practice and have the right sights to shoot at longer distances.