Look up on some posts on here that talk about modifying your stock air box, you can still make alot of power out of doing this to your airbox, Go out and buy a nice drop in air filter for it and people have cut them up and still made the car breath a lot better than it did. Also i know tuners say it is easier to tune a car with the stock air box that some cold air intake or something like that as well.
Ok, the links arent working on here for some reason. In the newbie section there is a link about CAIS and BOVs, check it out, it explains why they arent good, or worth much without more work than just bolting them on.
Ram air intakes usually pull in warmer air from the engine bay - even to some extent with a heat shield. Cold air intakes pulls in cooler air from the fender area. Cooler air can be compressed more/easier than warmer air when going through the same system. Your turbo system's main objective is to compress air and force more into your engine. Therefore, the cooler the air you start with, the better the system will function and the more power you are likely to make.
Those are the basics. But technically, a ram air intake system will breathe a little easier than the restrictive stock intake system (in most cases), so a ram air system is better than nothing just in terms of being able to allow more air into the system.
I dunno... The difference in IAT's between a ram air and CAI I feel should be negligable... The air is still going through a hot turbo after it gets sucked in... Maybe NA cars there is a bigger difference but FI should be a lot smaller margin. I'm not an expert though and for all I know the cooler air before the turbo does make a difference. Somebody needs to run an IAT test with stock airbox, CAI, and ram air setups and see if there is any noticeable difference in temps.
I know on the Suby system (because no one really makes a decent airbox) The stocker is good for some time BUT as a blanket statement it's a little far fetched. A guy on nastysock did an IAT test a while back.... I can't get on there from work (go figure).
For the record I was speaking of a traditional "ram air" which consisted of a scoop and a directed intake snorkel to attempt to "ram" air into the motor. This is the type I was saying would be of no benefit to a turbo car. A "short ram" (I don't know why they call them that) may offer some benefits on some vehicles. Especially in the sound department.
AEM CAI, Perrin ELH wrapped, Catted Downpipe, Stage II XPT tune, K4RT30Y STS, lip, shark fin