It appears that we (with 5.56) can harass them at long range better than they can harass us (with 7.62x39). Throw in some .303 and 7.62x54, and the complexion changes considerably. Still, one must be able to see relatively well at long range in order to put steel on target, as it were.
They do not need to see well at long range. They are typically firing at a group of men, or an outpost. They fire off some rounds that land within the general area and change locations.
They setup ambushes, ieds, various traps, etc
The following is some great insight into the type of fighting that goes on in the mountains and what weapons work well:
Here is a decent article on the Korengal Valley:
http://www.vanityfair.com/politics/features/2008/01/afghanistan200801?currentPage=1
Also on the Korengal Valley are several videos in this PBS story which is almost exclusively the soldiers in action without commentary. It shows how they try to win the support of the local elders and ties in well with the other article I linked. Creating ties with various local communities is a big part of current strategy.
http://www.pbs.org/frontlineworld/stories/pakistan802/iwitness/rubin.html
It is from a few years ago but relates to the type of fighting concerning the OP. Same terrain.
As far as calibers. What is ideal for longer range can leave soldiers more vulnerable in a close and medium range ambush. A lot of small cartridges allow lead sent in the direction of the enemy to last longer and fix them better.
Every soldier with a larger caliber weapon is one less soldier that is available to send large amounts of lead over time.
Heavier rounds weigh more, so they can carry fewer, and they will run out quicker suppressing the enemy.
The same tactics don't work as well with less ammo.
Everything is a trade off.
A lot of the most effective tools available to NATO forces can sometimes require suppressing the enemy for long periods of time. While aircraft are inbound, artillery is receiving information, or others are flanking or maneuvering.
So every fewer 5.56 weapon means they can last less time without additional supplies. Some fewer may increase performance in certain ways, but there is sacrifice.
That sacrifice is a reduced amount of time that the enemy can be fixed.
If you lose the ability to fix the enemy without quickly running out of ammo that means artillery and air power will be less effective, and flanking and various squad tactics will be much more difficult.
The enemy does win some victories with various remote outposts, and it was a big reason for the abandonment and retreat from many remote areas awhile back to focus on the more populated regions.
Or "strategic repositioning" I think they called it before the surge.
NATO has since pushed back into more similar areas.
This is a place that has a strong culture in insurgency and resistance going back hundreds of years. They celebrate defeating the British, the Soviets, and other battles.
With such a culture it would be more surprising if there was not a steady flow of new insurgent recruits when there is foreign occupation.
Even the definition and criteria for what will be considered victory is changing for US and NATO forces faced with this reality.
Talks with Taliban are being welcomed. Taliban are even being invited to join the government, the military, and the police forces of Afghanistan now.
Karzai just last month even threatened to join the Taliban a couple times. The Taliban have so much support he has to do such political posturing just to retain some legitimacy.
It has almost been a decade in Afghanistan, and it looks like it would take at least another decade.
I don't know if our nation will be willing to stay that long.
And when and if the Afghan government falls sometime after forces are withdrawn the replacement will likely be even worse than before. Much like what happened in Iran after the Shah fell.
You can only prop up a minority to rule over the majority for so long and pretend it is the will of the people, and when it falls and the majority regain control they hate you more than ever for it.
It appears we would have almost the same thing over there as before 9/11 not long after withdrawing, only more actively anti-US.
So there is a strong argument for staying there, but I don't see victory anytime soon unless they keep redefining what will be considered victory.