Couple things. First the only hi-flo cat available that bolts to the short headers is from Random Technologies and it cost as much if not more than the header, so cost wise, it's a bit much... Bosal is a few weeks(maybe longer) away from having their bolt on cat available, not sure how much it's going to cost.
Take a look at the FocusSport headers section, they publish their street header will give you 2-3 HP improvement, and their race header will add 6-7 HP on a "lightly modified" Focus.
Similar numbers should be expected on other street/short and race headers available.
Where you will pick up more HP is after you've done your exhaust mods(header, cat/no cat, flexpipe, catback), and you do adjustable cam gears. Tune them in and find lost HP.
Sound, you will definetly be louder with a race header. Short header and hi-flo cat, probably not as loud, but still louder than stock header, cat, and Borla...
Ask me again in a few weeks when I've put on my Ford Racing short header and Bosal cat, and flexpipe with the Magnaflow. Currently running just the Magnaflow.