Well, it sort of depends, but I have found SUL weights to be inadequate for comfort when on long multi-week trips, let alone long multi-month thru hikes. For example, the simple light I carry will need new batteries about once a week. I will need to carry extra's if there is no resupply shops that have them. Often, this is hard to determine ahead of time. An extra set of socks, some sort of good weather protection for two or three days of steady rains. And so on. I generally go UL for most trips, and don't try to reach the SUL weights. Generally, a 1# pack, 1# bag and 1# of shelter (all within a couple ounces) is pretty much required for all SUL trips. More comfortable hiking/camping starts when all of this stuff is closer to 1#8, but this will quickly leave little room for anything else with typical SUL gear. A fairly light weight 0C/32F system starts at about 6 pounds. I generally cannot aford all the cuben tarps, super expensive 900FP down jackets, etc, so I end up at about 8 pounds for the same system. If I am out longer than a week, I can expect this to reach 9-11 pounds. I need more comfort, and more reserve capacity. Even the NeoAir (at 13oz) will qickly break the SUL bank, but these old bones require it. Batteries and miscelaneous things(a more elaborate repair kit, a larger bottle of supper glue, 3-4 band-aids rather than 1-2, more duct tape, etc...) All this weighs. I never plan on needing this stuff. But, sheist will happen. I bring a cup. I don't for three days, simply living out of my pot. It is nice to be able to have coffee/cocoa AND eat breakfast and supper. I bring a third pair of socks. A set of long johns for sleeping in acts as reserve clothing for 32F weather. All these little things account for the difference between SUL and simply UL. In good weather, SUL is easy. As conditions get more iffy, you need more gear to cover your butt...well, at least to warm it up a bit.
Will Rietveld wrote a good series on Mountain SuperUltraLight a while back. Following his reasoning is about the same. It starts here: http://www.backpackinglight.com/cgi-bin/backpackinglight/mountain_sul_part1.html#.Ur255dJDuoU
I think he did 4 segments.