My car came with SSIs and a 2 in 2out Dansk muffler (70mm outlets).
BIG exhaust. It idled very low, and loped often. It poped and hissed off throttle, and made all sorts of noise when driving steady (because you're actually going ever so slightly on then off then on throttle).
I swapped the Dansk for a factory style 2 in 1 out, and the idle went up, smoothed considerably, and the a/f ratio went leaner (based on the burning eyes test
)
I'm sure that the PO never had the CIS retuned after putting that exhaust on.