Do you plan to measure your catalitic convertor temperature? If the temperature gets high enough the ceramic honeycomb inside will begin to erode. The erosion may partially block some of the honeycomb.
here is one of the possible part numbers for the matrix cat and front pipe from the factory:
174100D170
here is the price from 1st toyota parts $842.28
Though with the price of gas maybe your answer is "don't care".
Thats OK too.
