Absolutely love my 5500 and love this thread!
My K2 frame is all I've ever known as the first and only Exo I have bought was a 5500 in Fusion last year - a 2018 model. The pack is head and shoulders above anything else I've ever used, but over here in Australia, we don't get to try a whole lot of this sort of stuff often.
I reckon I'm keen on the 1800 day pack now and am stoked to read that the new packs are compatible with the old frame. I don't often load my 5500 up to capacity but it's good to know the space is there when I want to do more pack hunts. For now, I use it as a day pack that I intentionally load heavy to make my life difficult (keep me fit) and load out heaps of meat at once time. having a daypack that can load the same amount of meat out but be a smaller, streamlined pack for crawling around the scrub with a bow, gets me excited.
Keep up the good work, Exo. Love it.