Sorry, been hard at work over here...
The CEL might be from hitting the boost cut? Did you have a boost spike up in the 17psi range on a cold night or anything? If you hit the stock fuel cut for max boost (17psi) then you'll get a CEL. You can either wait (after a certain number of "trips" the CEL will reset itself unless you hit the boost cut again) or you can disconnect the battery's neg. terminal and step on the brake for 5 seconds to erase the ECU's memory (after which the car will have to relearn your driving habits, and you'll lose your radio stations! :shock: )
It sounds like your oil change just happened to coincide with the ECU deciding the CEL was a one time event and turning if off itself. To really know what's up, you need to have the codes pulled. I hear Pep Boys will do it for free, but I can't verifiy that. One of these days I'll get a code puller of my own, since (like Austin said) I'm constantly throwing CELs...
If you have a bad cat, you'll get the "catalytic converter inefficency" code which I believe is a P0420. Basically it means your exhaust is too cold according to the EGT, so it thinks your cat isn't working. I get this on my car, since my exhaust diameter is larger, the exhaust is cooler to the EGT. A simple resistor in place of the EGT sensor fixed that.
As far as the leak, I think Austin's right, it looks like you just had a leaky hose, fixed with a better clamp. Because of your STi radiator cap, you might be running a bit more pressure in there that may have caused the clamp to be insufficient.
FYI Austin, that picture shows the coolant resivoir that sits above the turbo and provides the coolant to the turbo. After the car's off, fluid siphons off that little tank to cool the turbo even tho the water pump's not running. The pressure valve (i.e. radiator cap) for the cooling system is on that little resivoir.