It takes dropping about 150+ lbs before you can actually notice any difference.
The general rule of thumb is that it takes about 20hp to really feel a difference in acceleration.
Stock focus zx3: 2500lbs(+160 for an average person, +60lbs in gas), 135hp
that makes about 20lbs per horsepower. So if we reverse that equation, we can say that if it takes 20hp to notice a difference, then it takes about the equivalent of 400lbs of weight loss to equate to a boost of 20hp in the engine bay.
That doesnt address the issue of performance, but I can say that gutting your daily driver isnt really worth it unless you go to the track often. Removing the back seat will have very little if any performance difference at all.
Also remember that your interior is also sound padding, gutted cars are obnoxiously loud to drive around in.