My 97 4.6 has 137k on the clock. I want to do a cat back system, but was also considering swapping to high flow cats. On my 4.6, I see two sets of cats and what looks like one set of 02 sensors? If this is correct, do I just whack both sets of cats and replace with one set? Any pricing on labor for this- from any one that has had it done. Also, how is the increase in power? Is it worth the expense