I would think pressure had absolutely nothing to do with those bbl's construction. Max chamber pressure has been traditionally measured at a point 1" from the breech, which ensured always being able to catch the highest peak. Those flares do not extend near an inch so would not assist in containing the max pressure. This construction is purely & simply to reduce the bbls weight while building it on a too large frame. The Stevens may have a more pleasing appearence in many's eyes but does so at the expence of an additionl 1lb 2oz of weight. A proportionately scaled 2 1/2" .410 would not exceed 4-4 1/2 lbs in weight. I have a Crescent built 28ga hammer double which weighs no more than this .410 & have used several 20gas over the years which went from 6 to 6 1/4 lbs. Many 16gas also do not exceed 6-6 1/2 lbs. Any .410 in excess of 5lbs is an aberration to me.
My helmet is on so blaze away if you desire, but to me putting a pip-squeak shell in a normal weight gun makes about as much sense as building a truck which weighs 5-6 tons empty & giving it a carrying capacity of a half ton.
