I own a 05 Focus with a 5/speed, They use the same engine as the 06. As far as speed, I was able to beat a modded '00 3.8 w/ a 47RW
trans. He was able to catch me after 70 because my car ran out of power as I got to fifth. However, when the car was swapped to a T-5, I was always able to get him off the line but by the time I reached third, he had caught me.
I've seen and help build some pretty fast 3.8's, there is a lot you can do to them. However if your looking for alot of HP, a GT is the way to go IMO.
Each has advantages and disadvantages.... it also really boils down to how much you want to spend to go fast. There was a 700rwhp 3.8 a while back. (I think that that record was broken recently.) And the limit of the stock internals has never been found, but he has some serious money tied up in that car.
Dunn