I have a 2003 focus st170 and have a 4-2-1 race manifold to go on and was going to put a scorpion race cat on and then a full scorpion back system on.Now people r saying I should just decat it that that would be better but what about the lambna sensors will they not put up the eml light and will that then not put correct fuel mix in and be heavy on fuel. Also when I change the manifold can I keep the heatsheld of so I can c the nice chrome manifold without having any heat problems in the engine. Thanks