I actually have a near zero curvature with my arc blast if it is fully loaded up, especially if I place my gear vertically rather than horizontally, which is what I typically do in these larger backpacks.
Here is a photo of mine, fully loaded up (minus water) for a hike. It includes SMD SX, ZP 20f bag, clothing, 5 days worth of food, tar xtherm, ditty bag, and the other usual stuff. (note: the umbrella is doing nothing for structural support)
This is loaded up with 15.8 pounds of gear and food.