My Gregory z30 is mesh backed and it does notably improve ventilation (30l capacity). The main issue is that the curve needed to create the air-gap doesn't so much lose space in the pack (although you do lose a bit as mentioned above) as make it more challenging for how you pack. In my case, the curve puts a large capacity space in the very bottom of the pack, with the mid-section getting narrower and then getting larger again at the very top opening. This means having to plan what goes where as opening up the U-zip for the main compartment sees the flap flop down (the flap has it's own pocket at the top, accessed from the outside, and anything heavy inside this means the flap really gets in the way if I'm trying to access the depths of the main space). The next issue is that I use a hydration bladder, which then has to fit around the curve of the frame, further adding to this mid-section bottle-neck problem. Air frames are also typically heavier packs due to the solid system for creating the curve.
I'd suggest working through exactly what you want to carry and whether you can get a larger pack than you might think you need to ensure you can access the contents without having to unpack to get into the deeper parts to the pack. This will be more expensive, however the other option is to figure out if you can manage your kit / hydration around the permanent curve of the pack.
Deuter are another brand to look at and whilst I don't think they do air-frame systems, they do have alternative designs that can be very light. I've been keeping my eye on a Deuter Aircontact Ultra 40l as it's very light with what looks to be good ventilation and great compression options to slim the pack down when not needing full capacity. Not going to be low cost anytime soon, but shows what's possible, and is still half the price of the extreme super-light packs on the market.
