Which year is your current Icon Pro 5200? Does it have the rubber anti-slip on the lower back area of the hip belt/pack area? (This is from the 2016 model of Icon Pro, I believe). Are you weighting the pack like was mentioned earlier? I like 65-70% on my hips and 30-35% on my shoulders with heavy loads.
My best guess is that you have don't have the pack properly fitted. I have used all of the Kuiu packs since they came out - beginning with the Icon. This includes carrying heavy loads in all of them.
Here is some of my history with Kuiu's packs...
In August 2013, I hauled a complete boned out Dall Sheep meat/cape/horns/camp/food/gear (125#+) out several miles with the first Kuiu Icon (It was my buddy's). It did pretty well.
In March 2014, I ordered and started using the Kuiu Ultra 6000 for about 5 months of training. It did OK, but is not designed for weight much above 75#+. The heaviest load I hauled with the Ultra was 135# of sand bags on a moderate trail (80' to 1,400' elevation gain in 1.5 miles, then back) around town. It was brutal and I really had to cinch down the waist belt to make it. The pack did the job but was not ideal.
In July 2014, Kuiu came out with the Icon Pro line and I was one of the first to order them. I put my Ultra 6000 bag on that pack, did some training, & went on several backpack hunts, carrying sheep (125-135#) and alpine deer (110-120#) loads. I enjoyed the lighter base pack weight, which is what you carry around 90% of the time on a hunt, anyways.
The Icon Pro has beefier hip belts and shoulder straps compared to the Ultra model, as well as the side-cinching waist belt straps (big plus for me). They still used the same carbon fiber frame, but carry loads much better with the new suspension. I have also ordered some of their newer (2016) models that have the rubberized lower back non-slip area (good addition) since. These packs are still quite light and can carry a pretty decent load without issues.
I have been using the Icon Pro suspension with the Ultra 6000, Pro 3200, & Pro 5200 pack bags for 3-1/2 years and feel that I am somewhat of an expert on using and fitting them (almost everyone I hunt with now owns them). I train year-around hiking 1-3 times a week starting in Dec/Jan carrying 25-50# on our local trails (~40' - 3,700' elevation gain). In February/March, we add snowshoes and/or lead for 40-80#+ packs depending on distance, snow depth, and elevation. In April/early May, we drop the lead weight and pick up rifles to hunt our local "hooters" (Sooty Grouse) on the mountain sides and avalanche chutes [Fun!]. Our pack weights are usually 35-50#.
After a little coastal black bear hunting in May, we are back training 2-3 days per week with increasing pack loads and longer distances (with the snow melting back). Our pack weights are usually 35-100# with occasional 130#+ trips. On some of the longer training hikes we cut back weight to 25-35# - as the distance and elevation gain can be brutal. One example: 17+ miles in 8+ hours with elevation gains of 3,900'+ in the first 2-3 hours with multiple 100'-900'+ descent/ascents (walking ridge tops) for the next several hours, followed by a large descent and hike out.
The heaviest Icon Pro pack I have hauled hunting was 2 boned out full-size Sitka Blacktail deer, some of my overnight gear, my rifle, and my bow/arrows out of the alpine 1,600 vertical feet for 1.5 miles. My pack weighed 152# and it took me 3.5 hours through the cliffs, scree, brush, muskeg, etc. The pack did very well.
However, no matter how you look at it - none of the Kuiu packs are quite up to the level/class of the Mystery Ranch Nice/Guide Light packs for true HEAVY load hauling (140-200#+). I have owned & used many Mystery Ranch packs (Kodiak, Nice Crew Cab, G-7000, Guide Light Marshall, & Guide Light Cabinet), as well as many of Dana Gleason's earlier packs (Dana Designs Terraplane, Astralplane, etc.) over the years and find that they are the "Cadillac" equivalent of internal frame packs. It rides very nice - especially under big loads. I would say the Kuiu Icon Pro is equivalent to a "Camry". For comparison, a Cadillac can comfortably and easily haul 6 people and ride very well. A Camry can haul 6 people, but not very comfortably and the ride will not be all that smooth (especially for the 6th person in the trunk...LOL).
I hope you give the Kuiu Icon Pro the benefit of the doubt and get it fitted properly. If you need some pointers, let me know your height, weight, torso length, frame and hip belt size of your current Icon Pro pack? For reference, I am 5' 9", 195#, 18-1/2" torso, Regular Kuiu Frame, Small/Medium Hip Belt (& MR Pack size Medium).
Good luck.
