I guess you don't have to use the side zippered pocket if you don't want to and that would leave more volume for stuff in the main side pocket.
Am I right in thinking that the all up volume of a large pack, ie main compartment and various outer pockets is likely to be 55L plus?
The target audience for this is, I guess, very unlikely to use anywhere near its weight carrying capacity and so are going to get a very comfortable pack for a c15lb base weight. Or to put it another way, if you are going to carry 45lbs you're not going to go looking for the lightest possible pack to carry it in.
For the zippered side pocket, the side zipper and top opening are competing for the same volume, so you want to be a bit thoughtful (e.g. you can't simultaneously jam it full from both openings). I usually do my fuel bottle in the stop and then there is still enough space for hats/mitts in the side.
I'm hesitant to put a total capacity including the external pockets because it's pretty subjective/hard to measure how much you can stuff into the external pockets but other companies typically count about 15L, so yeah it would be about 55L per that approach.
Yes if you actually carry 45 lbs (20kg) all the time you'd probably want to invest a bit more weight in the pack too. For example, even though the Kakwa 40 can handle it you might prefer something with larger/more thickly padded hipbelt so your hips have it easier. The target here is those 20-35 lbs loads (10-15 kg) where the dual strap/reverse pull hipbelt, load lifters, and horizontal frame structure all help to get it carrying nicely instead of pulling on your shoulders in odd ways.
Pretty.
Nicely thought-through.
I'm not a fan of shoulder pockets preferring a modular choice.
Wonder if all pocket elastics(?) are easily replaceable.
There are full daisy chains on the shoulder straps that run under the shoulder strap pockets, so it is quite easy (but committing) to simply cut off the shoulder strap pockets and put whatever style you prefer on there.
The pocket elastics on the side pocket and front pocket are not easily replaceable. We are sewing them 100% inside a hem of Ultra 200 to protect them from harm, and then they are not adjustable partly for that reason and partly because I think it's best/simplest for us to adjust the tension to be just right from the factory, instead of making it user adjustable with a cord lock, where it becomes more complicated/slightly heavier/more prone to damage.