Fitting a decat won't put the light on. But once it's fitted fit an upgrade walbro fuel pump. An air intake and a 3 port Solenoide and get the car re mapped
If I remember right... Fitting a decay on any model won't trigger a management light. But the 4 has a temp sensor that needs connecting to itself or it will throw a light up.
Your time will be fine.
Japspeed full stainless system is good for the price too