Well funny enough I'd never heard of the kestrel before? I just looked em up they seem pretty sweet! I bought the old brand trophy line tree saddle years ago and have been so happy with it I've never looked at any others. My saddle with the upper straps cut off basically looks just like the aero kestrel hunter without the back support. That being said I have no idea how the back suppport may add to the comfort or if it would possibly get in the way. I sit all day often in mine during the rut and it's all about how you set it up weather your comfy or not. The angle of the tree, how your steps, platform or branches feel etc... I wear knee pads and that is hugely important I think.
A lot of guys write these harness systems off I think because at the get go it seems complicated or weird but like so many things once you figure it out Its so simple and effective. In the end I use it because personally it makes me a better hunter. I can generally silently setup a tree and be hunting in minutes, when I'm in the tree I'm so hidden. I'm a big guy 6'7" in my saddle I stay tight up to the trunk and look like a big ol branch coming out at an angle haha. The way you can swing around the tree silently allows for getting on deer that get behind you or are moving through quick etc... on many occasions I've stalked into and area, setup a tree that seemed right in a situation that felt right, shot a deer from that setup and never hunted that tree again, it's awesome!