Even if you don't tune a Focus, the back end steps out a lot. That's because it's set to have a lot of snap oversteer.
Front wheel drive cars with very aggressive stock wheel sizes, like the Ford Focus RS and the Trial Celica, tend to have snappy rear ends (somewhat like in real life).
Unfortunately, GT4 doesn't seem to model rubber progression properly. I.E.: in real life, moving up to sports tires on a stock FF will give it a snappier rear end and a grippier front end. In GT4, this happens to some extent, but not as much as you'd expect... this has a lot to do with GT keeping the same profile tires on no matter what rubber you put on the car.
So while an Integra might feel generally snappier with race tires, it still exhibits some general understeer, which makes it easy to race with. The Focus RS, snappy to begin with, feels too quick to turn until you get used to it.
Generally the Focus RS suffers from tuning of any sort, as it's perfectly balanced as stock. The best you can do, I guess, is to soften the suspension tuning a little bit and dial down the power till you find the right balance. Some ballast may help, too, as the RS is a little too light for its own power. I've found the Focus SVT to be more tractable when fully tuned, for example.