Also posted over at LS1tech "Advanced Performance Tech":
http://www.ls1tech.com/forums/showthread.php?t=582091

I have been doing some research on catalytic converters and I also want to learn more from everyone that is knowledgable on the subject. What I have found so far:

1. There are two types of carrier/honeycomb/matrix available; ceramic and metallic. Metallic cats are the newer technology with many benefits including:
- Shorter light off time
- Less risk of overheating
- Less back pressure
- Larger effective area
- Greater catalytic capacity
- Smaller and more flexible design
- Thinner walls
- Higher mechanical strength
- Better resistance to thermal shock
Unfortunately the metallic carrier is much more expensive than the established ceramic technology.

3. Placement of the catalytic converter is important as you want it to get up to temperature quickly and maintain the temperature between 700-1000deg.F. If the converter is too cold, it will not do its job properly and if it is too hot, the converter will break down over time. If a mixture is too rich and fuel is being introduced into the exhaust, this fuel will burn in the catalytic converter resulting in temperatures over 2000deg.F which causes significant damage. Rich mixtures are much more damaging than lean mixtures (to the converter that is... a different story for your engine)

2. There are different emission standards that catalytic converters need to comply to. We are currently in the US Tier 1 bracket:
European standards over the years (g/km)
Tier.........Date...........CO.......HC. ........HC +NOx......NOx........PM
Euro 1......1992.........3.16.......-............1.13...........-...........-
Euro 2......1996..........2.2........-.............0.5...........-...........-
Euro 3......2000.........2.30.....0.20....... .....-...........0.15.........-
Euro 4......2005..........1.0......0.10...... ......-...........0.08........-
Euro 5....mid-2008.......1.0.....0.075...........-...........0.06........-

USA Standards (g/mile)
.................Date.........CO........ HC........ NMHC.......NOx.......PM
EPA Tier 1..................3.4........0.41...... .0.25..... ...0.4.......0.08
EPA Tier 2....2008+.....0.100......3.4.........0. 14........ -.........0.015

California standards (g/mile)
.........................NMOG.......CO.. .......NOx ........PM.......HCHO
Tier 1..................0.25.........3.4..... ....0.4... ....0.08.........-
LEV11.................0.075........3.4.. .......0.05........-........0.015
NMOG - non-methane organic gases
HCHO - formaldehyde

3. The carrier has different CPIS (cells per square inch) and is typically available with counts 100, 200, 300 or 400. Increasing the CPIS increases the surface area of the catalyst but also increases the pressure drop /length. To meet the emission standards however, a lower CPIS carrier will need to have a longer length (for same diameter casing), therby increasing the pressure drop anyway. From what I can tell, it is better to have a higher CPIS and a shorter length carrier. Alternatively a larger carrier diameter can help. Eg. I have seen one high-flow cat manufacturer has a carrier: CPIS=400 and 4"dia x 3"length for 2.5-3.5L motors (two cats for a 5-7L motor). I dont know what the aftermarket C5 & C6 exhaust systems (from various LSX exhaust suppliers) use.

4. In addition to CPIS and total carrier volume, the catalytic coating (palladium, platinum elements and rhodium) concentration can be altered by the manufacturer. These elements are VERY expensive and by applying a thinner coating, significant cost savings can be made. I think that some of the low cost LSX exhaust manufacturers might look at this first if they are trying to cut costs and lower their selling prices. Unfortunately, the thinner the coating, the less catalytic conversion and less likely that the converter will meet the emission standards (or last a long time).

So now we can see that not all catalytic converters are created equal. My guess is that the cheaper catalytic converters out there are built with lower cost carriers, lower carrier volumes, lower CPIS or lower catalytic concentration coating. All these factors will result in a catalytic converter that does not do the job properly or if it does do the job properly, will have a higher pressure drop than a more expensive catalytic converter.

As the saying goes (and this is especially true for cats), you get what you pay for. When in doubt, ask the manufacturer about the above. I'm done