[3BarBoost]

Actually, a CEL will come on if you delete the cat on any of the Stage kits (remember what I keep saying about passing the sniffer?). All the Stage kits run just fine with it in there, and cats don't degrade if you push more air through them. So doodling around town, the cat works just fine and keeps the tailpipe squeaky clean. Up top, the fuel goes open-loop anyhow and catalyst efficiency drops whether or not you have a Stage kit. Fortunately emissions isn't regulated up there - not that any car could stay squeaky clean at peak power, turbo or not.

If you were serious about power, and cared not about emissions compliance - a 2.25" straight through (race pipe) is more than fine with the stock turbo. However, a bigger turbo will want something bigger - the Stage 3 doesn't make any more power on the SRT with a 3". A PT installation might be different (because of the extra bends), but I think most 2.5" systems are fine. The O2 housing (what I call the downpipe) is not really that restrictive - it's something you'll change if you are after every last BHP, but your money is better spent upgrading to the Stage 3 turbo housing.