What headers did you get, are they coated? The O2 sensors may not be reacting quickly enough, not to mention, you went from a relatively restrictive y-pipe with cats to a free flowing one, so I'm sure that your fuel trims are off...

 

I Have Ceramic coated LT pacesetters. the check engine light is on but i knew that would happen until i get the rear o2s turned off.

 

See what codes you have, if there's more in there besides cat efficiency/overtemp/S2 codes, (such as S1 codes), that may be contributing to the problem...