Just installed downpipe and now have problems.

[90AWDTalon]

I just got my downpipe installed. It is 2.5" first bend the goes to 3" at the flex and is 3" the rest of the way back. After the install the boost builds much slower and power is way down. I logged a run and got a ton of knock, before the install I was getting very little knock. I think there might be an exhaust leak where it connects to the o2 housing. Also I'm not sure the guy rehooked the ground strap up. Could either one of these cause my problems.

 

[nineD5_GS-T]

do you have a gaskets on the downpipe for the cat and o2 flanges