What kind of AEM intake are you getting. Shorty or CAI. Shorty needs a heats shield, to much hot air. I had a AEM CAI, but sold it. I thought that the tubing was to small, and I didn't like the fact that the filter would get wet when it would rain hard. I also broke the mount for it on a hard down shift.
You can not upgrade you MAFS with the AEM. Possible future tuning drawback.
I was using the AEM CAI with a Magnaflow catback and Ess UDP. My AEM gave a nice pull in the mid-range through the upper rpm's. But I didn't think power up top was enough for what I lost off the line. 1st & 2nd felt slower, while 3rd and 4th pulled hard. The intake over powered my exhuast after 4k. I now have a Iceman upper intake pipe, add-on duct w/SVT snorkle. Feels like more torque, same mid range, but less top end then the AEM CAI.
The AEM CAI might feel better if you were to tune your stock cams (ie: move your power band). In my book AEM is not worth it for the Focus. Iceman/hybird or Volant is the way to go.
The Borla is a nice system, it's just a little loud for me. It's 2.5" piping is good for high HP all-motor, or F&I.
I went with the magnaflow b/c it's 2.25" and I'm only looking to make 150whp, 130wtq. The tone was just right for my taste. Not to loud just enough.
Those are just my views only. Most Intakes and Catback systems will give about the same results. It's all about 3 thing: how much money you want to spend, looks and the sound your after.
I would seriously think about an SVT airbox and snorkel instead of aftermarket. OEM quality and engineering is just (usually) much better than aftermarket - particularly when it comes to retaining drivability and weather proofing. However, I can not vouch that these parts are a drop in. I just got mine today (used to have AEM shorty, it was a POS) and plan to install tommorrow. I can tell you the air handling ability of the SVT box looks WAY superior to the original part. The snorkel is significantly larger and there is a good size secondary mini snorkel where the resonator is on the original box. The hole size is approximately 2-3 times that of the hole used for the original resonator box. If all goes well I will post on the install in the F/I forum tommorrow.
I have the Borla and have been quite pleased. Only thing is I would cut that ridiculous tip off and replace it with plain old SS tube before installation.