The downpipe and catalytic converter take exhaust gasses from the exhaust manifold on the engine, remove various toxins and noxious chemicals from the exhaust, and deliver them to the rest of the exhaust system (just in front of the resonator).

The OEM downpipe is known to be restrictive and causes sufficient back-pressure as to limit the engine's performance.  Hogan Performance used to fabricate an improved, mandrel-bent 409 stainless steel downpipe that significantly reduced this restriction and improves the engine's performance.  Hogan Performance's downpipe came with a Magnaflow high flow catalytic converter.  Installation was extremely easy with only four bolts to remove in order to swap out the original OEM downpipe and catalytic converter with an improved Hogan Performance downpipe and catalytic converter.  

Unfortunately, Hogan Performance is no longer fabricating replacement downpipes.  Some have had luck using a downpipe from ZZ Performance (http://www.zzperformance.com/); unfortunately, I haven't had the pleasure of using a ZZ Performance downpipe, so I can't comment on it.  They do advertise that they use 304 stainless steel (rather than the 409 stainless steel that the Hogan product used), which is a bonus.

I've use a Hogan Performance downpipe and Magnaflow catalytic converter on my Impala SS from spring 2010 - spring of 2011 when I upgraded to Doug Thorley headers.  I'm still using the Magnaflow catalytic converter, but the rest of the downpipe is long gone.  

The Magnaflow catalytic converter is part #94009; it's a 3" inlet, 3" outlet 3-way ceramic-coated converter with 200 cells per square inch (CPSI).  This converter is 49-state compliant, but it's not legal for sale or use in California (i.e., it's not CARB compliant).

I've been very impressed with the improvement and the craftsmanship of the Hogan Performance downpipe and Magnaflow catalytic converter.  I did notice, however, that after several months with the Hogan Performance downpipe and catalytic converter that the computer turned on the Check Engine Light. I was able to use my DiabloSport® Predator to read the error code -- it was PO420 which was the computer's determination that more air was getting past the catalytic converter than it was expecting.  Having more air getting through the system was the whole point of the upgrade so no surprise there!  I was able to contact 06MonteSS to modify my custom DiabLew Tune through my DiabloSport Predator Tuner to eliminate that warning and I haven't seen it since.

July 30, 2013 -- Hogan Performance DP7 downpipes (with catalytic converters) are available on eBay (link)