I dont think you would ever NEED cossie management on a 1.6 RST when running below the 280bhp mark as mentioned the 5th injector will do the job perfectly.

But for the price of a fully mappable 5th injector and having it mapped and fitted etc, your well on your way to paying for a full management change which is the route i took

If you do go cossie management.... Gadget is your man