I've followed chips for a few years and one thing I've found out is that almost ALL manufacturer gains are inflated.
They do this to sell products.
For a stock Zetec with an intake, exhaust and some minor bolt on's, look for around +5HP/TQ at the wheels.
If you have cams, induction, ported head and such, you can get about +10HP/TQ at the wheels or more. Usually more depending on how aggressive the mods are to the engine.
One thing they can do is give you more power/TQ eariler in the rpms but overall gain is usually a lot less as mentioned. This will give you a little kick off the line.
Just don't expect a hugh gain when you mainly have bolt-on's.
For ATX, you can have your shift points changed to shift at a higher RPM.
You can also get your Speed and Rev limiters increased but make sure you have the proper rated tires to exceeded 105mph.