I'll have to come out of the closet here and admit my fondness for the troop carrying Sea King (oh, the shame!)
I have no idea how the Sea King would perform in Afghanistan, but I've flown in the Royal Navy version lots as human cargo in many other types of conditions from the high arctic to temperate (and very intemperate) climates. Also in the Omani mountains. When it's properly kitted out for troop transport and flown by a competent crew, it rocks. In the arctic we regularly put 16 marines with the usual camping gear and weapons, plus 2 x toboggans and skis, in the back and bopped back and forth between sea level and about 6,000ft or so. They were the only ones who could find us and fly us out in definitely marginal (high winds, blowing snow) conditions. Their nav kit was excellent and it was small enough to put into just about any realtively small landing site, from an SF base helipad, small farmer's field or ship's deck, to a sub-alpine tree fringed meadow. It was large enough to lift a platoon/ troop in 2 chalks (or one lifts with two machines). They could also haul a moderate underslung load, like a 105 lt gun, apparently quite easily. Fast roping from a Sea King onto point targets was realtively easy, fast and safe. Parachuting from them in training was luxury compared with smaller models. Extra bonus points for being able to float if required. Now, the versions I worked with did not have armour, door guns etc. However, I see lots of S61s (civilian Sea King) in logging shows up and down the West Coast and they're stripped to the bones and hauling pretty big loads of logs quite easily at altitude. George Bush flies around in a Sea King. Helijet airways has a Sea King.
I guess the performance of the beastie just depends on how many $ you want to invest in your airframes. And if you had your choice between an old SK and a new support helicopter about the same size, would you still go for the SK? Not sure...
Once you've had a Sea King, you'll never go back to skiing for three days across two mountain ranges just because it's snowing.
OK, back in the closet with me.....