with sea foam in the vacuum smoke came out the top of heat shield?P0171code on 2004 Chevrolet Colorado

Just after I had glass packs muffler put in and cut off and turned down the following days had a reduce in power and little of a rough idle where's The best place to startlooking for tha problem?

Are the converter(s) still on and in original location? Changing backpressure will affect performance.