This is all pretty fascinating. At this point the only thing I'm sure of is that: A) I feel really fat seeing pics of someone cinching down a waist belt to damn-near it's minimum, and B) I wear my pack really low.
That said the medium Exos did look like it had the best shoulder pad fit but honestly I think it boils down to simply how it feels and carries. Everyone is different, really different. For me, they measured me in REI as a 19.5 but my large Exos is great because I simply can't stand wearing a hip-belt across the illiac ridge of my pelvis. Feels too high. Hurts my "slightly widened by fast food, size 38" gut. Places the load too high, which is to say, directly in line with where I have 3 herniated discs in my back. So as a result that ridge, for me, is about 1" higher than the top edge of the waist belt.
What I'm trying to say is, go hike. Do a couple overnights, see how things shake out. Maybe you'll be surprised. I kept trying to convince myself that, despite my body hurting and speaking to the contrary, that the the REI / forum folks MUST be right about hipbelt placement. Fast forward to me realizing I'm a unique and beautiful snowflake *sarcasm* and that if I break with convention a bit I'll get a better result, and now I'm thrilled with my (new) pack.