Wiims

to make it short...a week ago i got straight pipes put on my car. i didnt really notice a change in performance for anyone wondering. but i do like the sound its not for everyone. but anyways today i got my resonated tips in so i decided to get them welded on and since then in WOT my car seems to not accelerate until it hits 6 k and then it just takes off. it wasnt like that before the tips were on. so my question is could it be the tips are resonated? or the fact that the inlet is 2.5" and the outlet 4". the weld for the tips really look like just tac welds when i was expecting a weld all around the inlet of the tips. could that be the culprit? any help would be appreciatted.