Unless it is an actual race car, don't bother. Stripped down, gutted, race cars suck to drive as street cars, so how much do really plan on racing your Focus?
There will be such a small gain, it will require the timing equipment at the track to see the miniscule difference.
It will make your car less fun to drive, less enjoyable to own, less desirable to buy, it makes you look like a wanna be street racer if you get pulled over by a cop, etc.
There are so many downsides, and the upside is measured in hundredths of a second.
I took out the rear seats to make a cargo shelf type rear seat delete. I put the seats back in the next day, too much noise. Scrapped that plan.