Turbo Tubs are not legal in 944 spec. The reason is there is chance there might be additional stiffeness in the turbo tub. If we allowd it and some guys founnd an advantage it would be bad news to in effect for guys to run Turbo tubs to have "competitive car". We wrote the rules with this in mind and that is why we allow 944S tubs and 89 tubs we felt these would be similar enough in terms of 89 tub and for 944S there is no way to distingush the VIN number so if done right we would never notice.
I know for fact the 88 Turbo S and 89 Turbo tubs have additional chassis stiffening in them. It maybe possible to valided a turbo tub 944 spec used, but that would involved stripping both and NA and turbo tub from the same year and going over them closely to inspect for any changes. Metal thickness would also be critical to inspect. Given the supply NA donor cars that just does not seem a reasonable thing to do. Most of the time it just easier/cheaper to start with an NA tub.