Do you want a Greg Lowe clam shell pack like most tactical packs or traditional klettersak? Both have their merits but I prefer a klettersak due to a long slimmer main body that is less restrictive on the body and more weatherproof. Greg Lowe design is less weatherproof and is wider shorter, compact or squatty.
As far as volume, a lot depends on what goes inside. High speed light weight kit and I can get a week inside 28-33L. I carried 12.5 pounds for a 4 day push in the Gach with my dyneema WildThings Andinista pack. And this is the pack I recommend.
WT Tactical Andinista Pack | Lightweight alpine style climbing pack
If you are willing to spend $500 plus, give it a hard look. It can be zipped down to 1800ci or full open 4200ci. I have carried over 100 pounds in mine on 14 day trips that I guided.
Another pack I recommend in McHale. These are custom stitched for you, not off the shelf packs but a true custom pack to fit your body and needs about the same price as Kifaru.
Carrying weight is more than just being in shape. A pack that fits your body will carry more comfortably than one that does not. Grabbing an off the shelf pack, throwing it on the body and run to the hills is what most users do. But a pack fit to your body and packed correctly will offer more comfort.
For BOB only get a synthetic sleeping bag, around +20F.
Get trash compactor plastic bags to line your entire pack. Best weatherproof system I ever used.
I do not like kit hanging from the outside of my pack. Several reasons, it snags on things, produces more noise signature and can throw off your balance but I also know at times kit on the outside is a must.
Sounds like you are dedicated to Kifaru and there is nothing wrong about that, you asked so I am giving my opinion and expertise, take it or leave it, what ever you decide, run with it. Any pack in the 3000ci / 50L will be about perfect in volume, go up for winter or camping kit but 4000ci is about the volume limit for me.