It really comes down to preferences. For hunting deer and such, I'm totally satisfied with a plain old 4x. That's all I have on my black-powder rifles. Seriously, 200 yds is a pretty long poke for any muzzle loader, and if you can't see well enough with a 4x or 5x power, you just need to get closer. 4x at 200 is just like no magnification at 50 yds. Most people can shoot pretty good at 50 yds with open sights. Last thing I shot at 300 yds was a feral dog, and I shot it with a 30-06 and an Elite 3200 2-7x32 set on 3x.
Now of course, for shooting groups on paper, a higher power does make it easier to aim precisely. I'm all for that.