yes it sould go off. the CEL light is on because without the CATs the rear O2 sensors are reading that the cat (that arn't there) arn't working properly. and because of this the computer tells you something is wrong and trys to fix it by making your car run lean. you cang et some MIL elinators and this will fix the problem.
trys to fix it by making the car run lean?? where did you get that idea from? the rear o2 sensors simply monitor the efficiency of the cats and nothing more.
to the o/p, after you put the stock mid-pipe back on you will want to reset the computer to clear CEL because it takes a few drive cycles for it to go off on its own
i got that idea from local exhaust people. i have asked severail and they have all said the same soo thats what i believe. also i made myself some homemade MILs because my driver side rear O2 sensor went bad but i the cel keeps coming on and saying that there is a circuit problem would taht be because i made the mil wronh?? i know why the O2 is bad. the previous owner tried to gut the cats and stuck a screw drive down the pipe. and i think he hit the o2 and messed it up. but i'm wait itll my x-pipes come in to see if that y or not..