I think it really comes down to what you want as your "effective range". If it is 600 and under a 6.5lb rig with scope will do it as long as it is a quality setup and you practice. Now, if that required range is 1000 yds a heavier rifle will be offer more stability and therefore more accurate first round hits.
You just have to establish your goals and try to find a rifle that will help you meet them.
I have a goal of "sheep accuracy" at 600 yds. I have several rifles that will do it and built one specifically for it. Its a Rem Model 7 in 7 WSM with a scoped weight of 6.5lb. it will shoot sub MOA to 600 yds and thats all I expect it to do. The best part is it is really handy to carry around and has become my most carried rifle due to the weight and accuracy. This leads to more practice with it, which in turn makes me more proficient with it.
The other heavy rifles get carried still, but after 3 days carrying a 11 to 14 lb rifle, the lightweight sure gives a little relief when the body is sore and tired.
Now to the backpacking portion of the post. It seems if you get the lightest and best gear you can possibly afford (including the rifle) and GET IN SHAPE, you'l have nothing to cuss yourself about on the way down the mountain with a ram in your pack. I am in the process of gathering light, quality gear for a multi day backpack hunt in another part of the world and have found a few ounces here and there are important. Technology improves gear daily and weights continue to drop.
Here's a piece i came up with but haven't been able to test it yet. Maybe this year on a deer or elk hunt it will get some use?abuse to see if it will hold up. I think it will.
Three 270 WSM rounds.
A look at the rifle mentioned above and another I'm building for a friend in 264 Raptor that will be 6lb scoped when finished.
Now if someone would just figure out a way to dehydrate water we could all carry less weight!