I ran a decat when I first built up my 3" system but added the cat after a bit... didn't notice anything other than the exhaust was less boomy which is all good...
Std exhaust system and downpipe is most restrictive... the EGT's on my car dropped from over 1100 degrees to around 950 degrees when I went from std to the 3"... putting the cat on made no difference to EGT's which would suggest it in itself didn't present a restriction to the flow... if it was that much of a restriction the EGT's would have risen a bit.
3" DP, 200 cell cat and a decent free flowing exhaust will be as unrestrictive as you will ever need... should see spool kick in a tad earlier too
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