I've recently acquired a 2001 Corsa Milenium (a Brazillian saloon version of the Opel Corsa B, with a GM 8V 1.0 MPFI engine). It has a performance air filter (which I plan on replacing with the original filter soon) and a performance exhaust. I noticed on reductions, when I'm using engine braking or decelerating that there's some backfiring in the exhaust.
From some quick research I've found lots of candidate reasons for that, including ignition timing being a little off, valves adjustment slightly opened, amongst others. But I also read that "Decel Pop" is a known issue with performance exhausts1,2, since it's natural that some fuel mixture will come out in the exhaustion during a deceleration, but with a performance exhaust more air reaches the inside of the exhaust pipe and the rich mixture makes it burn and make that poping sound.
- Is it really normal that some fuel comes out during the exhaustion step?
- Should I be worried about exhaust backfiring on deceleration considering the car has a performance exhaust installed?
 http://www.allthingsmoto.com/forums/f-13/deceleration-popping-explained-10/ (This considers a motorcycle with a carburator, but I believe it's an issue with electronic injection cars as well?)