single turbo RB30(25)DET only requires normal road worthy and emmissions test for legal rego (according to SA statment of requirments) but RB30(26)DETT would require bigger brakes, weight to power ratio test, etc. given that both engines look stockish with all the extra bits that do nothing but look like they make it economical.
A single turbo three liter twin cam is considered to be the same as a VL turbo and R32 gts-t has better brakes than a VL so it passes with little testing, but a twin turbo is considered to be more like a GTR so more requirments to get legal.