You can always put camera gear in to a serious hiking backpack but it's hard (if not impossible) to take an expensive photography backpack and use it for serious hiking.
I could not agree more. I've been using a F-stop Loka for a few years now, and while it's a great backpack and probably one of the finest camera backpacks available, I feel a lot more comfortable with my Arcteryx Astral 65 even if I put twice as much weight in it. There is a world of difference, at least to my me and my back. I would say that camera backpack manufacturers have a very long way to go, when it comes to ergonomics. I'm thinking of adding a Lowepro Toploader AW 50 to my kit, now that I have sold the Loka, to be chucked in with the rest of my hiking kit. It should hold a L-plated 6D with a wide angle zoom nice and snugly.
Generally I agree that trekking/mountaineering-specific packs are the best available in terms of wearing comfort. But the comparison of the F-Stop Loka with the Arc'teryx Altra is not really fair. The Loka is a 37 litre pack with not that much padding on the hip belt and the shoulder straps. It is not meant to carry 30 kilos like the Altra (which is a fabulous pack by the way). If you try a Tilopa BC (48 litres) or a Satori EXP (62 litres) you might get a rather different experience: both packs are very well constructed and padded so that you can carry your 20-30 kilos of photo gear, mountaineering stuff, clothes, food etc. quite comfortably. It's still heavy to lug around 20 kilos though…