i don't think frame connectors were left off to save money.If the engineers felt they were a benefit a simple change in the die could have added 2 more folds in the floor.I'm betting the stock stamping matched the strength of the unibody ahead and behind it.
As for twist, adding torque boxes would provide a gusset type re-inforcement and there is nothing so strong as a triangle, but adding frame connectors seems to be adding 2 more 2x4's to a stud wall which it seems wouldn't help much.
...i just tryin' to look at things logically.
