In my view the additional tube does not really help. it is putting a beam *inside* another beam. As an engineer that is the most hopeless place to add mass; in the center of the element! It is however easy; weld it in & forget. It also helps tremendously when you're hit from the side by a low nosed car like a Ferarri.
I considered it also; I even bought a roll cage quality tube for it but choose not to go that route. It's easily 15 kg of mass added to the car; too much for me.
Also the corrosion aspects of such a modification are tricky. The inside of such a tube and between the tube and inner sill it can hardly be protected effectively by DIY techniques; only an acid dip and paint dip will work for such enclosed volumes.
The sill itself isn't really weak I think. what makes the body weak is that the various stiff parts (Vertical A-pillar, frame rails, tunnel, sills, firewall, front struts) aren't rigidly connected to each other. Our e9 chassis are missing triangle shaped links between these various components, we have just nearly flat sheets of 0,8 mm metal. Think of forces flowing through a body following the lines that are visible in tubular designs as the Maserati birdcage and 300SL.
Besides what is already mentioned, i see a few places on an E9 body that could be improved in my view:
- top of tunnel (around gear shift) to corners of the upper firewall/ upper a-pillar (but there is a lot of stuff in the way).
- frame rails to sills, directly connecting lower A-pillar to the frame rail near the firewall, and another triangle hiding under seat structures. (serious 3D metal crafting)
- the rear diff mount, as done for instance with the Coupe King solution.