EDIT it was me - I didn't read and thus my post didn't apply ( i assumed you had a standard ek1 !

The only relevance is the o2 sensor location - and the possibility that it was damaged / disconnected in the new install of your car/collector for the new exhaust.
I suggest the o2 sensor is located directly after the collector , and may of been damaged or the wiring to it damaged when installed. Get it checked out by the shop