I'm considering a CAI or short ram for my Zetec/ZX3.
I had a big concern about AEM, as my local perf shop had one customer complaining about sucking water. I also have a '93 Civic, which has the stock air intake located low behind the front bumper and it sucks very dirty air - filters last just a few months.
So short ram looked like the way to go, but there are concerns of air temperature. So, I did a Mythbuster's experiment. I hooked up a electronic remote thermometer taped at the position of air intake of a short ram, and a CAI. At warm idle (18C ambient temp), there was a 15C higher temp at the short ram location versus the CAI position. But at cruising speed, there was a 2C difference, and at highway, no measurable difference.
So, the temp problem with short intakes is only a problem at idle/low speed. Which may effect drag runs, but not an issue for me. Whether or not even 15C makes a real difference, I don't know.
What I rarely see, and makes much more sense, is a small hood scoop to draw outside air in from a short-ram intake, with a foam water barrier.
Lastly, I asked five manufacturers for the data on horsepower increases they claim from 5-13 hp. Not one could provide me with the actual data.
Steeda has a dyno run on the website with a 6 hp gain. But from a single Dyno run, which is meaningless. You can bet that that was the best gain ever, and practically, it's below that. I doubt 4% will make a noticeable difference other than engine sound. I'm certain these dyno runs were not done with accurate air flow fans, which means they were testing at low air flow, which will magnify any effect.
I don't undertstand Ebay claims of better gas mileage, more flow means more power, more power means more gas used.
I'm not kidding myself, I'll fit one 'cause it sounds cool.
As for suggested Focus performance inprovements #1-#4..
#1 tires (I really like my Fuzions)
#2 suspension (I've got FR SVT kit-recommend)
#4 anything that lowers weight, the Focus is pudgy.