I'm a new Kestrel owner and I don't have nearly the hours that a lot of guys have, but I'm pretty sure that comfort depends on a ton of variables. Yeah size is important, but bridge length, tether height, tether length, amount of lean/sit, knee pad quality, platform or ring of steps and what steps they may be and how they attach, your individual body type and fitness, and even your mental attitude all interact to effect comfort. And I think just as important is developing a polished system. Fumbling with the rig each time will lead to frustration, anxiety and loss of enjoyment.
I bought a size 1 and immediately tested it with my warmest layers. It feels great. No complaints about the fit or adjustments. But like most guys, I've tweaked a few things, and I'm still not done tweaking.
I'm very happy overall with my choices.