I'm not sure I'm edumacated well enough to answer all your questions but I can share these pics of Kirkham's Stainless steel chassis.
Yes they are Ladder frame, birdcage frames, as you can see in the pics, however I believe they use a thicker gauge of steel for chassis rails, etc, so aren't as flexy as originals.
Kirkham does offer a billet chassis, most probably stiffer than anything, but that costs more than most could afford.
They keep the weight down by using Aluminium throughout the car, as opposed to fiberglass, or steel (ie: suspension pieces, or door, hood, truck frames, etc).
I cant advise on Backdraft or Factory 5, but ERA chassis is built differently, using square tube and not round tube, some extra bracing, to make their frame stiff, but the small compromise is weight. In my experience, Stiffness is more important that weight.
The theory would suggest less weight would be faster, but if it aint suitably stiff, then it wont be able to get the traction required to pull the lateral Gs